U.S. patent number 5,278,687 [Application Number 07/681,128] was granted by the patent office on 1994-01-11 for multiwavelength data communication fiber link.
This patent grant is currently assigned to Physical Optics Corporation. Invention is credited to Tomasz P. Jannson, Richard C. Kim, Behzad M. R. Moslehi, Kevin W. Shirk.
United States Patent |
5,278,687 |
Jannson , et al. |
January 11, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Multiwavelength data communication fiber link
Abstract
An optical multi-channel data communication link transmits data
from a transmitter to a remote receiver. The link includes a
multimode laser diode source connected to the transmitter, a device
for modulating the source with each channel of data to generate a
modulated light wave, and a wavelength division multiplexing device
at the transmitter. The wavelength division multiplexing device has
paraxial optics for multiplexing the light waves to produce a
multiplexed signal. Multimode fiber optic means are connected to
the multiplexer and pass the multiplexed signal to the remote
receiver. A wavelength division multiplexer has paraxial optics for
demultiplexing the multiplexed signals at the receiver to produce
demultiplexed light waves. Each of the demultiplexed light waves
are converted for use by the receiver. Preferably, each of the
wavelength division multiplexer and demultiplexer has a littrow
reflecting grating and a lens which are paraxially aligned with the
multimode fiber optic means to provide channel separation of less
than 50 nm.
Inventors: |
Jannson; Tomasz P. (Torrance,
CA), Shirk; Kevin W. (Redondo Beach, CA), Moslehi; Behzad
M. R. (Redondo Beach, CA), Kim; Richard C. (Yorba Linda,
CA) |
Assignee: |
Physical Optics Corporation
(Torrance, CA)
|
Family
ID: |
24401215 |
Appl.
No.: |
07/681,128 |
Filed: |
April 5, 1991 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
599816 |
Oct 18, 1990 |
|
|
|
|
Current U.S.
Class: |
398/79;
398/87 |
Current CPC
Class: |
H04J
14/02 (20130101); G01J 3/44 (20130101) |
Current International
Class: |
G01J
3/44 (20060101); G02B 6/34 (20060101); H04J
14/02 (20060101); H04J 014/02 () |
Field of
Search: |
;359/124,129,130,131,193,8,575,615,569 ;385/123,42,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Chilcot, Jr.; Richard E.
Assistant Examiner: Bacares; Rafael
Attorney, Agent or Firm: Niles & Niles
Parent Case Text
This is a continuation-in-part of Ser. No. 599,816 filed on Oct.
18, 1990.
Claims
We claim:
1. An optical multichannel data communication link for transmitting
data from a transmitter to a remote receiver, said link comprising
a laser diode source coupled to said transmitter; means for
converting each channel of data to a light wave from said source;
wavelength division multiplexer means, at the transmitter, having
paraxial optics for multiplexing the light waves to provide a
multiplexed signal; fiber optic means, connected to the multiplexer
means, for passing the multiplexed signal to the remote receiver;
wavelength division demultiplexer means having paraxial optics for
demultiplexing the multiplexed signal at the receiver to produce
demultiplexed light waves at the receiver; and means for
reconverting each of the demultiplexed light waves for use by the
receiver; said wavelength division multiplexer means and
demultiplexer means having fibers, a littrow reflection grating,
and lens means which are paraxially aligned with said fiber optic
means, said link being optimized to minimize cross talk by one
of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
2. The multichannel data communication link defined in claim 1
wherein the means for modulating comprises a laser diode per
channel within a single window of 750 to 850 nm wavelengths and
channel separation of 10 to 30 nm.
3. An optical multichannel data communication link for transmitting
data from a transmitter to a remote receiver; said link comprising
a source connected to said transmitter; means for converting each
channel of data to a modulated light wave from said source;
wavelength division multiplexer means, at the transmitter, having
paraxial optics for multiplexing the light waves to produce a
multiplexed signal; multimode fiber optic means, connected to the
multiplexer means, for passing the multiplexed signal to the remote
receiver; wavelength division demultiplexer means having paraxial
optics for demultiplexing the multiplexed signal at the receiver to
produce demultiplexed light waves at the receiver; and means for
reconverting each of the demultiplexed light waves for use by the
receiver; said wavelength division multiplexer means and
demultiplexer means having a littrow reflection grating and lens
means which are paraxially aligned with said multimode fiber optic
means to provide channel separation of less than 50 nm; wherein the
means for converting comprises an LED or an edge emitting LED
having wavelength separation, .DELTA..lambda., and linewidth,
.delta..lambda., such that
.delta..lambda./.DELTA..lambda..ltoreq.2(1-d/b), where d is the
fiber core diameter and b is the cladding diameter.
4. The multichannel data communication link defined in claim 1
wherein the means for converting comprises a laser diode per
channel within a single window of wavelengths near 1300 nm and
channel separation of 10 to 30 nm.
5. The multichannel data communication link defined in claim 1
wherein the means for converting comprises an edge emitting LED per
channel within a single window of wavelengths near 800 nm and
channel separation of 10 to 30 nm.
6. The multichannel data communication link defined in claim 1
wherein the means for converting comprises an edge emitting LED per
channel within a single window of wavelengths near 1300 nm and
channel separation of 10 to 30 nm.
7. The multichannel data communication link defined in claim 1
wherein the means for multiplexing comprises a wavelength division
multiplexer having a broadband reflection grating with uniform
wavelength characteristics.
8. The multichannel data communication link defined in claim 1 or
claim 6 wherein the means for converting and means for reconverting
are analog processing circuits.
9. The multichannel data communication link defined in claim 1 or
claim 6 wherein the means for converting and means for reconverting
are digital processing circuits.
10. The multichannel data communication link defined in claim 1 or
claim 6 wherein the means for converting and means for reconverting
are digital processing circuits for a number of the channels and
are analog processing circuits for the remaining channels.
11. The multichannel data communication link defined in claim 1
wherein the means for converting comprises a laser diode per
channel within the dual wavelength windows of 1310 nm and 1550 nm,
respectively.
12. The multichannel data communication link defined in claim 1
further comprising electronic feedback compensation means for
reducing temperature sensitivity of the link.
13. The multichannel data communication link defined in claim 1
wherein the fiber optic means comprises a multimode fiber having a
core/cladding ratio of 50/125, 62.5/125, 80/125, 100/140, 200/380,
or other core/cladding ratios close to 0.5, preferably within the
range: 0.3-0.85.
14. The multichannel data communication link defined in claim 1
wherein said fiber optic means has at least two fibers of equal
diameter so that substantially all total internal reflection (TIR)
angles are filled during operation so that modal partition noise
and sensitivity to mechanical disturbance are minimized.
15. The multichannel data communication link defined in claim 1
wherein the means for reconverting comprises a photodetector for
each channel.
16. The multichannel data communication link of claim 15 wherein
the photodetectors are located at the output of the wavelength
division multiplexer means and connected directly thereto.
17. The multichannel data communication link defined in claim 1
wherein the link transmits three channels which are converted to
750 nm, 780 nm, and 810 nm wavelengths.
18. The multichannel data communication link defined in claim 1
wherein the multiplexer means comprises a surface relief
grating.
19. The multichannel data communication link defined in claim 1
wherein the multiplexer means is a photoresist grating.
20. An optical multichannel data communication link for
transmitting data from a transmitter to a remote receiver, said
link comprising a laser diode source coupled to said transmitter;
means for modulating said source with each channel of data to
generate a modulated light wave; wavelength division multiplexer
means, at the transmitter, having paraxial optics for multiplexing
the light waves to produce a multiplexed signal; fiber optic means,
connected to the multiplexer means, for passing the multiplexed
signal to the remote receiver; wavelength division demultiplexer
means having paraxial optics for demultiplexing the multiplexed
signal at the receiver to produce demultiplexed light waves at the
receiver; and means for reconverting each of the demultiplexed
light waves for use by the receiver; said fiber optic means
comprising a single-mode fiber; said wavelength division
multiplexer means and demultiplexer means having fibers, a littrow
reflection grating, and lens means which are paraxially aligned
with said single-mode fiber optic means, said link being optimized
to minimize cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
21. The multichannel data communication link defined in claim 20
wherein an optical delay line per channel compensates for
interwavelength fiber dispersion.
22. The multichannel data communication link defined in claim 20
wherein the single-mode fiber has a core/cladding ratio of
9/125.mu..
23. The multichannel data communication link defined in claim 20
wherein the wavelength division multiplexer means comprises a
Littrow volume holographic grating which preserves
polarization.
24. The multichannel data communication link defined in claim 20
wherein the means for modulating comprises a distributed feedback
laser.
25. The multichannel data communication link defined in claim 24
wherein the number of channels exceeds 20 and further comprising
means for limiting cross-talk to less than -20 dB.
26. An optical multichannel data communication link for
transmitting data from a transmitter to a remote receiver, said
link comprising a multimode laser diode source coupled to said
transmitter; means for converting each channel of data to a light
wave from said source; wavelength division multiplexer means, at
the transmitter, having paraxial optics for multiplexing the light
waves to produce a multiplexed signal; multimode fiber optic means,
connected to the multiplexer means, for passing the multiplexed
signal to the remote receiver; wavelength division demultiplexer
means having paraxial optics for demultiplexing the multiplexed
signal at the receiver to produce demultiplexed light waves at the
receiver; and means for reconverting each of the demultiplexed
light waves for use by the receiver; said wavelength division
multiplexer means and demultiplexer means having fibers, a littrow
reflection grating, and a lens which are paraxially aligned with
said fiber optic means, said link being optimized to minimize cross
talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
27. The multichannel data communication link defined in claim 26,
the lens replaced by a collimating mirror.
28. The multichannel data communication link defined in claim 26,
wherein the lens is a GRIN lens.
29. The multichannel data communication link defined in claim 26
wherein the fibers comprise an input fiber and a plurality of
output fibers, all said fibers being in alignment.
30. The multichannel data communication link defined in claim 26
wherein the input fiber is not in alignment with the output
fibers.
31. The multichannel data communication link defined in claim 1
wherein the fiber optic means comprises a single-mode fiber wherein
the fiber parameters of core diameter b and cladding diameter d,
fiber loss, length L, numerical aperture NA, and tolerance dx; the
light source parameters of source power P; frequency dv and
wavelength control d.lambda. and wavelength shift coefficient
k.sub..lambda. ; the grating parameters of grating dispersion,
coefficient K.sub..lambda., Littrow angle .alpha., resolution
1/.LAMBDA., grating constant .LAMBDA., DCG refractive index n, and
grating optical density O.D.; the lens parameters of focal length L
and diameter D; signal dispersion; and general link parameters of
power budget, power margin BER, data rate per channel and aggregate
data rate are all optimized to minimize cross-talk.
32. An optical multichannel data communication link for
transmitting a multichannel signal from a source to a remote
receiver, said link comprising transmitter means, optical path
means, and receiver means;
said transmitter means comprising a laser diode source, means for
converting each channel of the signal to a light wave from the
source, and wavelength dispersive means for multiplexing the light
waves;
said optical path means transmitting at least as many wavelengths
as the number of channels and connecting the transmitter means to
the receiver means;
said receiver means comprising wavelength dispersive means for
demultiplexing the light wave and detector means for detecting each
of the demultiplexed light waves and reconverting the light waves
for use by the remote receiver;
said wavelength dispersing means having fibers, a littrow
reflection grating, and lens means which are paraxially aligned
with the optical path means, said link being optimized to minimize
cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
33. An optical multichannel data communication link for
transmitting a multichannel signal from a source to a remote
receiver, said link having a known wavelength shift, d.lambda., and
comprising transmitter means, optical path means, and receiver
means;
said transmitter means comprising a source, laser diode means for
modulating said source with each channel of the signal to generate
a light wave, and wavelength dispersive means for multiplexing the
light waves;
said optical path means transmitting at least as many wavelengths
as the number of channels and connecting the transmitter means to
the receiver means;
said receiver means comprising wavelength dispersive means for
demultiplexing the light wave and detector means for detecting each
of the demultiplexed light waves and converting the light waves for
use by the remote receiver;
said wavelength dispersive means having fibers, a littrow
reflection grating, and lens means which are paraxially aligned
with the optical path means to provide channel separation of less
than 50 nm
said link optimized so that the b/d ratio for said fibers is set
equal to (b/d).sub.c wherein (b/d).sub.c is determined from the
equation (b/d).sub.c =2-.zeta..sub.d where b is the core to core
distance between fibers and d is the diameter of the fiber cores,
and wherein the (b/d).sub.c equation is solved for by substituting
maximum loss, .zeta..sub.max, from the equation ##EQU60##
34. An optical multichannel data communication link for
transmitting a multichannel signal from a source to a remote
receiver, said link comprising transmitter means, optical path
means, and receiver means;
said transmitter means comprising a source, laser diode means for
modulating said source with each channel of the signal to produce a
modulated light wave, and wavelength dispersive means for
multiplexing the light waves;
said optical path means transmitting at least as many wavelengths
as the number of channels and connecting the transmitter means to
the receiver means;
said receiver means comprising wavelength dispersive means for
demultiplexing the light wave and detector means for detecting each
of the demultiplexed light waves and reconverting the light waves
for use by the remote receiver;
said wavelength dispersive means having fibers, a littrow
reflection grating, lens means which are paraxially aligned with
the optical path means to provide channel separation of less than
50 nm;
said link having a predetermined minimum dispersion loss,
.zeta..sub.d, in accordance with the equation ##EQU61## where
.zeta..sub.D is a function of b and d, and b is the core to core
distance between fibers, d is the diameter of the fiber core, and k
is the wavelength tolerance shift.
35. The multichannel data communication link defined in claim 32
wherein the laser diode means comprises standard laser diodes
having wavelengths centered at 750, 780, 810, or 840 nm.
36. An optical multichannel data communication link for
transmitting a multichannel signal from a source to a remote
receiver, said link comprising transmitter means, optical path
means, and receiver means;
said transmitter means comprising a laser diode source, means for
converting each channel of the signal to a light wave from said
source, and wavelength dispersive means for multiplexing the light
waves;
said optical path means transmitting at least as many wavelengths
as the number of channels and connecting the transmitter means to
the receiver means;
said receiver means comprising wavelength dispersive means for
demultiplexing the light wave and detector means for detecting each
of the demultiplexed light waves and reconverting the light waves
for use by the remote receiver;
said wavelength dispersive means having a littrow reflection
grating and lens means which are paraxially aligned with the
optical path means to provide channel separation of less than 50
nm;
wherein the optical path means comprises fibers having a fiber
geometry meeting the condition b/d=(b/d).sub.c, wherein b is the
distance between adjacent fiber cores, d is the diameter of the
core of the fiber, and (b/d).sub.c =1/(1-k.sub.o) where ##EQU62##
where .DELTA..lambda. is the center wavelength separation of each
light wave and d.lambda..sub.c is the maximum permissible
wavelength variation from the center wavelength of each light
wave.
37. The multichannel data communication link defined in claim 36
wherein (b/c).sub.c is adjusted so that the tradeoff between
cross-talk and insertion loss is minimized.
38. An optical multichannel data communication link for
transmitting data from a transmitter to a remote receiver, said
link comprising a laser diode source coupled to said transmitter,
means for converting each channel of data to a light wave from said
source; wavelength division multiplexer means, at the transmitter,
having paraxial optics, for multiplexing the light waves to produce
a multiplexed signal; fiber optic means, connected to the
multiplexer means, for passing the multiplexed signal to the remote
receiver; wavelength division demultiplexer means having paraxial
optics for demultiplexing the multiplexed signal at the receiver to
produce demultiplexed light waves at the receiver; and means for
reconverting each of the demultiplexed light waves for use by the
receiver; said wavelength division multiplexer means and said
wavelength division demultiplexer means comprising a wavelength
division multiplexer/demultiplexer having paraxially aligned
fibers, a lens, and a littrow reflection grating to provide channel
separation of less than 50 nm, said fibers of said multiplexer
having a b/d ratio different than the b/d ratio of said fibers of
said demultiplexer, where b is the core to core distance between
fibers and d is the diameter of the fiber core, said link being
optimized to minimize cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where k=d.lambda./.DELTA..lambda., where d.lambda. is
the maximum acceptable wavelength shift of said laser diode source
and .DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
39. An optical multichannel data communication link for
transmitting data from a transmitter to a remote receiver, said
link comprising a source connected to said transmitter, means for
modulating said source with each channel of data to generate a
modulate light wave; wavelength division multiplexer means, at the
transmitter, having paraxial optics for multiplexing the light
waves to produce a multiplexed signal; fiber optic means, connected
to the multiplexer means, for passing the multiplexed signal to the
remote receiver; wavelength division demultiplexer means having
paraxial optics for demultiplexing the multiplexed signal at the
receiver to produce demultiplexed light waves at the receiver; and
means for converting each of the demultiplexed light waves for use
by the receiver; said wavelength division multiplexer means and
said wavelength division demultiplexer means comprising a
wavelength division multiplexer/demultiplexer having paraxially
aligned fibers, a lens, and a littrow reflection grating to provide
channel separation of less than 50 nm, said fibers of said
multiplexer having a b/d ratio different than the b/d ratio of said
fibers of said demultiplexer, where b is the core to core distance
between fibers and d is the diameter of the fiber core, wherein the
respective b/d ratio of the multiplexer and demultiplexer is due to
a difference in the thickness of the cladding between the fibers in
the multiplexer and demultiplexer respectively.
40. A multichannel data communication link for transmitting data
from a transmitter to a remote receiver, said link comprising means
for conditioning the signal from the transmitter, said means for
conditioning being connected electrically to the transmitter, laser
diode means for converting the signals output from the signal
conditioning means, said laser diode means being connected to the
signal conditioning means via one optical fiber per channel,
wavelength division multiplexing means, at the transmitter, for
multiplexing the outputs of the laser diode means and connected to
the laser diode means via one optical fiber per channel, wavelength
division demultiplexing means connected to the output of the
multiplexing means via an optical fiber, detector means connected
to the demultiplexing means via one optical fiber per channel,
signal conditioning means connected electrically to the outputs of
the detector means, and receiver means electrically connected to
the outputs of the signal conditioning means, said multiplexing and
demultiplexing means having paraxially arranged transmission
optics, fibers, and a littrow broad band uniform reflection
grating, said link being optimized to minimize cross talk by one
of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
41. The multichannel data communication link defined in claim 40
wherein edge limiting LED means is substituted for the laser diode
means.
42. A multichannel data communication link for transmitting data
from a transmitter to a remote receiver, said link comprising means
for conditioning the signal from the transmitter, said means for
conditioning being connected electrically to the transmitter, laser
diode means for converting the signals output from the signal
conditioning means, said laser diode means being connected to the
signal conditioning means via one optical fiber per channel,
wavelength division multiplexing means, at the transmitter, for
multiplexing the outputs of the laser diode means and connected to
the laser diode means via one optical fiber per channel, wavelength
division demultiplexing means connected to the output of the
multiplexing means via an optical fiber, detector means connected
to the demultiplexing means via one optical fiber per channel,
signal conditioning means connected electrically to the outputs of
the detector means, and receiver means electrically connected to
the outputs of the signal conditioning means, said multiplexing and
demultiplexing means having paraxially arranged transmission optics
and a littrow broad band uniform reflection grating wherein the
optical fibers connecting the laser diode means and the
multiplexing means are of smaller diameter than the optical fiber
connecting the multiplexing and demultiplexing means, and the
optical fibers connecting the demultiplexing means and the
detectors are of larger diameter than the said fibers and fiber,
said link being optimized to minimize cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
43. A multichannel video link for linking the central processing
unit of a real time market data computer to a plurality of data
feed monitors and keyboards, said link comprising a transmitting
module which fits into the electronics rack of the computer, a
receiving module which fits into the electronics rack of a remote
receiver to which is connected the plurality of data feed monitors,
and optical path means connecting the transmitting module to the
receiving module, said transmitting module comprising a laser diode
source for each channel in the system operating in the single
window range of 750-850 nm, a wavelength division multiplexer
connected to each of the laser diode outputs and having fibers, a
uniform broadband reflection littrow grating, and paraxial
transmission optics; said optical path means comprising a standard
multimode fiber; the receiving module comprising a wavelength
division demultiplexer (WDDM) connected to the optical path means
and having fibers, a uniform broadband reflection littrow grating,
paraxial transmission optics, and an output for each channel, and
photodetector means connected to each output of the WDDM, the
outputs of the photodetector means connected to the data feed
monitors, said link being optimized to minimize cross talk by one
of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
44. A multichannel video link for linking the central processing
unit of a real time market data computer to a plurality of data
feed monitors and keyboards, said link comprising a transmitting
module which fits into the electronics rack of the computer, a
receiving module which fits into the electronics rack of a remote
receiver to which is connected the plurality of data feed monitors,
and optical path means connecting the transmitting module to the
receiving module, said transmitting module comprising a laser diode
source for each channel in the system operating in the single
window range of 750-850 nm, a wavelength division multiplexer
connected to each of the laser diode outputs and having a uniform
broadband reflection littrow grating and paraxial transmission
optics to provide channel separation of less than 50 nm; said
optical path means comprising a standard multimode fiber; the
receiving module comprising a wavelength division demultiplexer
(WDDM) connected to the optical path means and having a uniform
broadband reflection littrow grating and paraxial transmission
optics to provide channel separation of less than 50 nm, and an
output for each channel, and photodetector means connected to each
output of the WDDM, the outputs of the photodetector means
connected to the data feed monitors, wherein the wavelength
division demultiplexer is optimized in accordance with the
following equation ##EQU63## where b is the core to core distance
between fibers, d is the core diameter, and k is ##EQU64## wherein
d.lambda.0 is wavelength shift.
45. A multichannel video link for linking the central processing
unit of a real time market data computer to a plurality of data
feed monitors and keyboards, said link comprising a transmitting
module which fits into the electronics rack of the computer, a
receiving module which fits into the electronics rack of a remote
receiver to which is connected the plurality of data feed monitors,
and optical path means connecting the transmitting module to the
receiving module, said transmitting module comprising an LED for
each channel in the system operating in the single window range of
750-850 nm, a wavelength division multiplexer connected to each of
the laser diode outputs having a uniform broadband reflection
littrow grating and paraxial transmission optics to provide channel
separation of less than 50 nm; said optical path means comprising a
standard multimode fiber; the receiving module comprising a
wavelength division demultiplexer (WDDM) connected to the optical
path means and having a uniform broadband reflection littrow
grating and paraxial transmission optics to provide channel
separation of less than 50 nm, and an output for each channel, and
photodetector means connected to each output of the WDDM, the
outputs of the photodetector means connected to the data feed
monitors, wherein ##EQU65## where b is the core to core distance
between fibers, d is the fiber core diameter, and
k'=.delta..lambda.,.DELTA..lambda., .delta..lambda. is ELED
linewidth, and .DELTA..lambda. is wavelength separation.
46. An integrated services digital network (ISDN) comprising a
first plurality of connected computer workstations, a second
plurality of connected computer workstations, a multiplexing link
having electrical inputs from the first plurality, and video and
voice signal inputs and an optical output; and a demultiplexing
link having an optical input connected via a single fiber to the
optical output of the multiplexing link, and an electrical output
to the second plurality and video and voice signal outputs; said
multiplexing link comprising a laser diode source; wavelength
division multiplexer means coupled to said source and having
paraxial optics including first fibers, lens means, and a littrow
reflection grating paraxially aligned with said first fibers; said
demultiplexing link comprising wavelength demultiplexing means
having paraxial optics including second fibers, lens means, and a
littrow reflection grating paraxially aligned with said second
fibers, said link being optimized to minimize cross talk by one
of
(A) setting a b/d ratio for said first and second fibers so as not
to be smaller than (1/1-k), where b is the core to core distance
between fibers, d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying, for said first and second fibers, the equation:
k<1-d/b, where k=d.lambda./.DELTA..lambda., where d.lambda. is
the maximum acceptable wavelength shift of said laser diode source
and .DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
47. A local area networking (LAN) system comprising an engine
workstation connected to wavelength division demultiplexing means
(WDDM); said WDDM connected optically via a single transmission
fiber to remote wavelength division demultiplexing means (WDDM)
connected to a remote workstation; said engine workstation
transmitting signals to and receiving signals from said remote
workstation over said single transmission fiber, said WDDM and
remote WDDM having fibers, a littrow reflection grating, and lens
means paraxially aligned with said transmission fiber, said link
being optimized to minimize cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
48. The LAN system defined in claim 47 wherein the engine
workstation transmits standard red-green-blue (RGB) signals to said
remote workstation and transmits to and receives data signals from
the remote workstation.
49. The LAN system defined in claim 47 wherein the WDDM means
comprises fibers, a lens, and a reflection grating all arranged
paraxially.
50. The LAN system defined in claim 47 wherein the remote
workstation comprises a standard computer monitor and a keyboard,
the monitor receiving said standard red-green-blue (RGB) signals
and the keyboard receiving and transmitting said data signals.
51. A multichannel data communication link comprising a first
wavelength division multiplexing bidirectional coupler, a second
wavelength division multiplexing bidirectional coupler, first and
second transmitters including a laser diode source, and first and
second receivers, the first transmitter inputting a first
wavelength light wave into the first coupler, the second
transmitter inputting a second wavelength light wave into the
second coupler, the first coupler inputting the first wavelength
light wave into the second coupler and the second coupler inputting
the second wavelength light wave into the first coupler, the second
coupler outputting the first wavelength light wave into the second
receiver and the first coupler outputting the second wavelength
light wave into the first receiver, said couplers having fibers, a
littrow reflection grating, and a lens paraxially aligned, said
link being optimized to minimize cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said laser diode source and
.DELTA..lambda. is wavelength separation, and where d is the
diameter of the fiber cores and b is the core to core distance
between fibers.
52. The multichannel data communication link defined in claim 51 in
a local area network, metropolitan area network, or wide-area
network.
53. The multichannel data communication link defined in claim 51
wherein the first and second couplers receive and transmit the
first and second wavelength light waves between each other via a
single mode fiber.
54. The multichannel data communication link defined in claim 51
wherein the first and second couplers receive and transmit the
first and second wavelength light waves between each other via a
multi-mode fiber.
55. The multichannel data communication link defined in claim 51
wherein the couplers comprise a volume holographic grating in
quasi-Lippman geometry, lens means for collimation and
concentration, and first, second, and third fibers, all arranged
paraxially.
56. The multichannel data communication link of claim 51 wherein
the first and second transmitters comprise single mode, 3-channel
wavelength division multiplexers.
57. The multichannel data communication link defined in claim 51
wherein the first and second receivers comprise single mode,
3-channel wavelength division multiplexers.
58. The multichannel data communication link defined in claim 51
wherein the first wavelength equals about 1320 nm and the second
wavelength equals about 1510 nm.
59. A broad band source comprising a paraxially aligned lens,
littrow reflection grating, and first, second and third fibers,
said first fiber being an output fiber, said second and third
fibers being input fibers carrying two separate wavelength light
waves slightly shifted from each other from two separate LEDs, said
two separate wavelength light waves being multiplexed and coupled
into said first fiber by the lens and grating with wavelength
separation of less than 50 nm between the two light waves, said
source being optimized to minimize cross talk by one of
(A) setting a b/d ratio for said fibers so as not to be smaller
than (1/1-k), where b is the core to core distance between fibers,
d is the diameter of the fiber cores, and
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said LEDs and .DELTA..lambda. is
wavelength separation, and
(B) satisfying the equation: k<1-d/b, where
k=d.lambda./.DELTA..lambda., where d.lambda. is the maximum
acceptable wavelength shift of said LEDs and .DELTA..lambda. is
wavelength separation, and where d is the diameter of the fiber
cores and b is the core to core distance between fibers.
60. The broad band source as defined in claim 59 wherein the light
waves are generated by ELEDs.
61. The broad band source as defined in claim 59 further comprising
means for coupling at least 50% of the total energy of the two
separate wavelength light waves into the first fiber.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to the transfer of data signals
point to point. More particularly, the invention relates to
transfer of data between source and receiver wherein the data in
the form of an electrical signal is converted into light waves,
transferred via a fiber optic cable to a receiver and then
converted back into an electrical signal.
2. Description of the Prior Art
Transmission of multiple data signals from one point to another can
be accomplished in a virtually unlimited number of ways. Transfer
of information can occur over different frequencies (optical,
microwave, & radio) and media including air, twisted wire, coax
and more recently, fiber optic cables. Examples of data
transmission in air include television and radio. Transfer of
information by wire could include, for example, macroscopic
structures such as coax or twinax cable for carrying a television
signal from a receiving antenna to a television set or a cable
connecting a peripheral unit such as a printer to a personal
computer, to microscopic structures such as the minute electrical
paths that guide electrons in an integrated circuit. Examples of
transmission of data using fiber optic cables include fiber optic
telephone cables. In each of these types of systems it is typically
desirable that data should be transferred between source and
receiver as quickly as possible and with utmost accuracy and
maintenance of desired signal characteristics.
Two broad categories of transmission of data would include
transmission by analog and digital means. Within each of these
types of transmission, numerous modulation techniques of data have
been used including FM, AM, and pulse modulation (PM). Techniques
providing means for not only modulating a carrier with information
from one signal but for multiplexing numerous signals for
transmission together include time division multiplexing (TDM), and
frequency division multiplexing (FDM). FDM in essence involves
stacking a number of data channels side by side in the frequency
domain to form a composite signal. The composite frequency
multiplexed signal is used to modulate a carrier in a conventional
manner. TDM is a derivative of pulse modulation and involves
interleaving in time the narrow pulses of several pulse modulated
signals to form one composite signal. Separation of the TDM pulses
at the receiver is accomplished by directing the pulses into
individual channel filters.
In recent years, as a result of the maturation of fiber optic
technology in transmission systems, advances have been made in a
multiple carrier technique referred to as wavelength division
multiplexing (WDM). This technique is the optical equivalent of
frequency division multiplexing used in RF coaxial transmission. In
WDM, each discrete data channel is modulated onto an optical
carrier of a fixed wavelength and each of the carriers are then fed
into the optical transmission medium. The individual carriers are
recovered at the receiver by separating the carrier into its
individual wavelength components. One such example of a WDM is
disclosed in U.S. Pat. No. 4,926,412, the essentials of which are
incorporated by reference herein. There, a WDM is disclosed having
paraxial transmission optics.
A transmission optics WDM differs from a reflection optics system
in that in the former, the light beam changes direction only once
in the system; it is first collimated by a lens, diffracted by a
grating, and then focused by the lens into the fibers. In paraxial
optics, the input and output optical fibers are maintained close to
the optical axis to limit losses in the system due to dispersion
broadening, and image aberrations.
In a reflection optics WDM, the beam changes direction three times;
the beam is first collimated by a mirror, diffracted by a grating
located in roughly the same plane as the fiber ends, and focused by
the first mirror into the fibers. Reflection optics WDMs, produced
by Instrument SA and others, are based on relief surface
holographic gratings, usually coated on curved surfaces that
simultaneously perform imaging operations. Other types of WDMs,
produced by Kaptron, for instance, utilize spherical imaging
reflection surfaces, usually coated with multilayer dielectric
films to provide wavelength selective reflectance (see, e.g., F.
Unterleitner, Fiberoptic Product News, Vol. 5, No. 12, November
1990, p. 27). An entirely different WDM approach based on biconical
coupler technology, is proposed by Gould Electronics. Still another
WDM system, based on lens transmission optics and highly
non-uniform flat volume holographic gratings, has been proposed by
Physical Optics Corporation in the above mentioned U.S. Pat. No.
4,926,412. References relating generally to optical fibers and
fiber communications are plentiful and include for example: S.
Miller and I. Kaminow, Optical Fiber Telecommunications (Academic
Press, 1988); and D. Baker, Fiber Optic Design and Applications
(Reston, 1985)incorporated by reference herein.
Without question, transmission of data optically versus by wire
means is becoming more and more prevalent. Numerous reasons exist
for using optical transmission of data, as opposed to electrical
transmission including bandwidth limitations, electromagnetic
interference, weight and bulk. In order to achieve high bandwidth
in an electrical data transmission system, the wires must have
large diameters for shielding from EMI and consequently are bulky
and heavy. Furthermore, power losses associated with data
transmission over electrical ires are very large, and signal
repeaters must be placed at relatively small intervals (500 m) even
for low frequencies (<10 MHz).
Transmission of data optically, on the other hand, can provide huge
bandwidth characteristics, extremely low loss even over long
distances and immunity to electromagnetic radiation even in
environments saturated with electronics such as aircraft.
Furthermore, because the bandwidth of an optical fiber actually
increases with a decrease in the diameter of the fiber optic cable,
huge bandwidths (>300 MHz for 1 km length of multimode fiber)
are obtainable with extremely light and nonbulky transmission
lines.
Although the majority of LANs are still coax-based or "twisted
pair", video transmission systems, which are more demanding than
LANs because of higher bandwidth requirements and remote location
desirability are better suited to fiber optics. Typical high speed
fiber optics systems are red-green-blue (RGB), closed circuit
television (CCTV), and computer aided design (CAD) RGB and CCTV are
typically analog with a 10-50 MHz throughput while CAD is digital
or analog with 120-300 MHz throughput. Typical multi-mode fibers
used for data transmission have 50/125.mu., 62.5/125.mu.,
100/140.mu., and 200/380.mu. core diameters. Fibers having these
four core/cladding diameters are standard in most fiber optic
applications and their cost of manufacture continues to decrease.
Note that in a state of the art single-wavelength design only the
core diameter is important because the core contains the traveling
beam to the exclusion of the other portions of the fiber such as
the cladding.
The advantages of optical transmission of data have not escaped
industries where transfer of data is critical to transaction of
daily business. Video local area networks (LANs), i.e., video
conferencing, security systems, and the securities brokerage
industry are three examples where transfer of information by fiber
optic cable has been in place for sometime. Fiber optic cables have
been used to transfer data between a video camera and a security
alarm processor in high end security systems. In another
application, information is transferred between a main computer
that keeps track of market conditions and the tens of screens in a
securities trading room.
In the future, Integrated Services Digital Networks (ISDN) which
may provide 3-channel information for homes and businesses
(telephone, video and data) will require high quality transmission.
These systems, and Broadband Integrated Services Digital Networks
(BISDN) will become commonplace.
In securities trading applications, typically called data feed
terminal systems, the three video components (red, green, and
blue), are continuously fed to trading room video screens in
basically two ways. One means of transfer currently in use is
inputting each of the three components of the video signal from the
source computer into a light emitting diode (LED), or laser diode
(LD) which converts each of the electrical signals to a light
signal which is then fed, via a dedicated, separate optical path,
to the trading screen. Another slightly newer and less common
approach is to first multiplex (using TDM) the three electrical
components of the video signal and then feed the multiplexed
electrical signal to a LED, or LD for conversion to a multiplexed
light wave which is then fed to the trading room video screen and
reconverted to three electrical signals. Most such systems in use
today are standardized around the RS 170, and RS343A standard for
computer generated video signals.
The major drawback of the first system is that it requires the use
of three separate modules in the electronics rack and, most
importantly, requires three lengths of fiber optic cable to be run
from the central computer to the trading room screens potentially
many floors below. This obviously creates size and cost
constraints. The major drawback of the slightly newer
implementation of data transfer (TDM) is that the signals must be
electronically multiplexed and then fed to a laser which must
convert the multiplexed electrical signal accurately into a
multiplexed light wave and transmit it to the video screen.
Committing one light source to the task of converting a multiplexed
electrical signal into a multiplexed light signal is less than
desirable because it is typically a low efficiency conversion. The
bandwidth of the fiber also becomes an issue in this format because
200.mu. core fiber cannot effectively transmit the bandwidth of an
RGB signal on a single wavelength due to modal dispersion. Length
is an issue as well; the signal can be broadcast but only over very
short distances.
Still another disadvantage of TDM technique is that the multiplexed
signals must be of the same modulation format, usually digital. To
the contrary, WDM fiber-optic systems can multiplex, through
various wavelength carriers, not only different format signals such
as digital and analog, i.e., RS170 & RS232, but also various
types of information related to different wavelengths, specific to
a particular sensing medium such as in Raman spectroscopy, for
example.
There are three basic types of light sources used in optical fiber
data transmission, surface light emitting diodes (LEDs) and edge
limiting LEDs (ELEDs), and laser diodes (LDs). Surface emitting
LEDs have been in use for many years in many different
applications. They are extremely reliable and relatively
inexpensive. Laser diodes, on the other hand, are a much more
recent technology, are slightly less reliable than LEDs, and are
usually more expensive. LDs, however, as well as ELEDs have certain
advantages over LEDs, that are becoming consistently achievable as
LD technology matures in the compact disk (CD) industry. LDs are
well known in use as the light source for reading CDs in now quite
common CD players. The market for LDs created by the CD industry is
large and has caused the development of standard LD wavelengths
located in the 1st transmission spectral window: 750-850 nm. Within
this range, Sharp has developed standard LD WDM wavelengths 750,
780, 810, 840, all in the vicinity of the CD wavelength 780 nm.
Siemens, Phillips, Hitachi and Ortel also make LDs. Because of the
huge production of LDs in these standard wavelengths, LDs have
become extremely low cost ($10-$30) and price competitive with
LEDs. Recently, ELED technology has achieved maturity, with a
typical unit price of around $100. ELEDs' wavelengths, on the other
hand, are usually located in the 2nd transmission window, around
1300 nm.
The primary advantage of LDs over LEDs is that LDs have much
narrower spectral characteristics. Furthermore, LDs are much faster
than LEDs. It is difficult to achieve 200 MHz with LEDs, while LDs
can obtain greater than 1 GHz bandwidths. Additionally, the life of
a typical laser diode is 250,000 hours or 120 years assuming it is
not abused with high current or physically damaged. Also ELEDs have
significantly narrower linewidths than surface-emitting LEDs,
typically 50-100 nm versus 100-200 nm.
Unfortunately, with respect to transmission of multiple channels
from source to receiver, TDMs and FDMs require very troublesome and
sophisticated electronics while multi-fiber solutions are expensive
and difficult to implement in space tight applications. Therefore,
a data transmitting system that does not require the use of TDM or
FDM multiplexing nor multiple fibers would be of great benefit and
cost saving for all data transmission applications.
SUMMARY OF THE INVENTION
A multiwavelength data communication link employing low cost LDs,
ELEDs and/or distributed feedback (DFB) lasers, holographic
wavelength dependent dispersive grating elements, multimode or
single mode optical fibers and associated electronics is presented.
Specifically, low cost LDs within the single transmission window
750 nm-850 nm, as well as around 1300 nm, are modulated with the
data to be transferred. A paraxial grating WDM, resistant to
adverse effects from even substantial wavelength shift, may
multiplex the separate modulated laser diode light beams for
transmission on a single fiber optic cable to a receiver comprising
a similar WDM (also called WDDM) which demultiplexes the
multiplexed data into the original data channels. Electronic
circuitry converts the individually modulated light waves into
their electrical counterparts and then feeds the channels to the
receiver. In another embodiment, the optical signal is modulated by
a sensing medium, before it is transformed into an electrical
signal.
Low cost GaAs LDs manufactured in the most common wavelength ranges
and particularly 780 nm standard in the CD industry may be used as
sources. GaAs LDs in the 750-850 nm window are particularly low
cost and may act as the carrier for a separate channel of
information (or more than one channel if TDM or FDM is employed) to
be transferred independently and with extremely low cross-talk (or
optical isolation), and nearly full transparency. The LDs receive
their respective data channel information from electronics which
adapt the channel in electronic form to the LD for conversion to a
light wave modulated with the information. Each of the modulated
light waves is then space-multiplexed in a fiber cable using
wavelength division multiplexing techniques.
A preferred WDM comprises paraxial optics and a holographic
dispersive element, preferably a broadband reflection quasi-Littrow
grating, which assures extremely high multiplexing efficiency. The
fiber optic cable on which the information is sent is a multi-mode
fiber capable of handling the number of multiwavelength channels.
The fiber optic cable therefore has extremely high total bandwidth,
multiplying the bandwidth capacity by the number of wavelengths
multiplexed. Fiber optic cable diameters standard in the industry
may be used further lowering cost. The multiplexed signal in the
fiber optic cable is received by another similar WDM (WDDM) which
demultiplexes the signal into discrete signals corresponding to the
original data channels. Again, the low loss and cross-talk of the
WDM having paraxial optics and a holographic dispersive element
makes the demultiplexing operation highly efficient. The
demultiplexed modulated light waves may then be fed to three or
more photo detectors which convert the light waves back into
electrical signals. The converted electrical signals are then fed
to a receiver such as a data feed terminal or video monitor. It
should be emphasized that demultiplexing (WDDM) is more critical
than multiplexing (WDM) because WDM can be accomplished in a number
of alternative ways with good power budget, while only paraxial
WDDM can separate wavelengths with high efficiency and low cross
talk.
The above combination provides the ability to transmit a number of
independent (multiwavelength) channels (analog & digital)
within a single transparent window less than 200 nm wide with
nearly full transparency and extremely low cross talk. The present
invention enables voice, video, analog, or digital signals to be
transferred point to point inexpensively and with a minimum of
fiber length, equipment, and bulk. Importantly, the present
invention therefore has a variety of applications, is low in cost,
especially due to the ability to use current CD LD technology and
low cost electronics, a variety of standard fiber types and lengths
including lengths greater than 1 km with excellent power budget,
high tolerance to wavelength shift, low cross talk, and full
transparency. These highly optimal results, heretofore
unobtainable, are achievable due to a combination of paraxial
transmission optics, optimized WDM design, high quality highly
uniform and broadband wavelength reflection holographic gratings
and either multiwavelength low cost single window operation CD LDs,
rapidly maturing ELEDs, or DFB lasers.
The optimized WDM design permits use of the present invention in
multi-mode and single mode fiber optic applications where DFB
lasers, conventional LDs, or LEDs are employed in either single or
multi-window operations. The present WDM system geometry provides
benefits resulting from the unique and unexpected relationship
(both mathematical and structural) between the link's parameters
such as insertion loss, cross-talk, signal dispersion, and system
size; and WDM system design parameters such as source linewidth,
source wavelength shift, wavelength separation, grating dispersion,
grating optical density, fiber core/cladding ratio, fiber
core/cladding geometrical tolerance, fiber insertion loss, fiber
dispersion and lens focal length.
DESCRIPTIONS OF THE DRAWINGS
FIG. 1 is a block diagram of a video system in accordance with the
present invention;
FIG. 2 is a schematic of the electro/optic circuit modules shown in
FIG. 1 in accordance with the present invention;
FIG. 3 is a schematic of a WDM grating configuration of a link in
accordance with the present invention;
FIG. 4 is a graph depicting AC power budget;
FIG. 5 is a schematic of the transmitter and receiver portions of a
link in accordance with the present invention;
FIGS. 6A & 6B are schematics of an ISDN system and a remote
work-station system in accordance with the present invention;
FIGS. 7A-7D are a schematic of various WDM architectures of the
present invention;
FIG. 8 is a graph plotting source linewidth, or, alternatively,
wavelength shifts, versus grating spectral characteristics;
FIG. 9 is a graph plotting source wavelength shift, d.lambda.,
linewidth .delta..lambda. and wavelength separation
.DELTA..lambda.;
FIG. 10 is a graph plotting efficiency .zeta. versus k;
FIG. 11 is a graph plotting efficiency, .zeta. versus
d.lambda.;
FIGS. 12A and 12B depict fiber light spot misalignment;
FIG. 13 illustrates a rectangular ID model for cross-talk
calculation;
FIG. 14 illustrates cross-talk optimization, according to Eq.
14;
FIG. 15 illustrates cross-talk optimization, according to Eq.
18;
FIGS. 16A and B show the WDM filtering effect of the present
invention;
FIG. 17 shows the relationship between fibers and their wavelengths
in of the present invention;
FIGS. 18A-D show the WDM filtering effect of the present invention
using only one light source;
FIG. 19 is a schematic of a multiple external modulator system of
the present invention;
FIG. 20 is a schematic of a "smart skin" sensor system of the
present invention;
FIGS. 21A-B are schematics of multi-wavelength sensors of the
present invention;
FIG. 22 is a schematic of WDM optical isolation structure of the
present invention;
FIG. 23 is a schematic of a bidirectional WDM grating splitter of
the present invention;
FIG. 24 is a schematic of a security camera WDM system in
accordance with the present invention;
FIG. 25 is a schematic of a bidirectional, dual-wavelength, single
mode data link of the present invention with extremely low
cross-talk;
FIG. 26A is a schematic of a WDM employing a quasi-Lippman Littrow
volume holographic grating and
FIG. 26B is a plot of the grating's reflection characteristics,
O.D. versus .lambda..
FIGS. 27A-D are schematics of a single mode WDM, WDDM and
bidirectional dual wavelength coupler, and dual window WDM cascade
of the present invention.
FIG. 28 is a schematic of a single mode WDM link of the present
invention having WDM bidirectional couplers and WDM
transceivers;
FIGS. 29A and B are schematics of a WDM based local area network
(LAN) for communicating data, voice, video, and sensor
information;
FIG. 30 is a schematic of a dispersion-compensation WDM system of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Multiwavelength Data Communication Link
Referring now to FIG. 1, a multiwavelength data communication link
10 comprising transmitter 12 and receiver 14 is depicted.
Transmitter 12 may comprise signal source 16, signal conditioning
circuitry 18, laser diode driver circuitry and diodes 20 (which may
alternatively be ELEDs), and multiplexing wavelength dispersive
means 22. Signal source 16 may Comprise any multichannel signal
such as video, computer, telephone or other data channels, or
multi-color RGB signal. Signal conditioning circuitry 18 conditions
and adapts the signal for input into the laser diode driver
circuitry and laser diodes 20. Laser diode driver circuitry and
diodes 20 receive the conditioned signal and output that signal in
light wave form to multiplexing wavelength dispersive means 22, for
exemplary wavelengths .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3 as shown in FIG. 1.
Each channel is provided a dedicated line for transfer between
signal source 16, signal conditioning circuitry 18, and laser diode
driver circuitry and laser diodes 20, and the multiplexing
wavelength dispersive means 22. The laser diode driver circuitry
may comprise standard circuitry that provides the proper voltages
for operation of the laser diodes. Such circuitry may comprise for
instance a standard biasing circuit to bias the LD above threshold
and below kinks in the current vs output curve, and a standard
op-amp circuit for adjustable scan of the input. The laser diodes
in block 20 may preferably comprise standard laser diodes used in
the CD industry having standard operating wavelengths of 750, 780,
810, and 840 nm. Sharp laser diodes may preferably be used, but for
higher speed links (>20 MHz per wavelength), other LDs or ELEDs
may be more convenient. These LDs are low cost because the
thickness of the aluminum needed to be doped into the LD for this
wavelength range (particularly 780 and 810 nm) to achieve proper
band gap energy and wavelength can be accomplished without unduly
stressing the GaAs layers which bound the aluminum doped layers in
the liquid phase epitaxial growth (LPE) process or MOCVD (Metal
Organic Chemical Vapor Deposition) process.
Multiplexing wavelength dispersive means 22 may comprise any device
which multiplexes light signals according to wavelength. In the
preferred embodiment, the multiplexing wavelength dispersive means
may comprise a highly paraxial WDM having a broadband reflection
grating with highly uniform and high diffraction efficiency
wavelength characteristics. Less desirable but operable gratings
include transmission gratings and mirror based reflection gratings.
Preferred high diffraction efficiency gratings may include DCG
volume holographic gratings and photopolymer volume holographic
gratings, if the photopolymer has sufficiently broadband uniform
wavelength characteristics. Some types of photoresist gratings and
substrate-etched grating structures having above average
diffraction efficiency and broadband width uniform characteristics
may be used as well. Particularly advantageous gratings would
include highly vertically-nonuniform volume holographic DCG
gratings wherein nonuniform alcohol-water processing techniques are
used to create a grating having an extremely flat efficiency curve
over the wavelength range of interest. Such a highly nonuniform
volume holographic grating is fully described in U.S. application
Ser. No. 435,608 the essentials of which are incorporated by
reference herein as well as in: T. Jannson et al., Opt. Soc. of
Amer., Vol. 8, No. 1, p. 201 (January 1991).
The transmitter 12 is connected to the receiver 14 via multimode
fiber 24 which may comprise any of the standard multimode fibers
presently manufactured. The relative core to cladding diameter
ranges for these standard fiber sizes are: 50/125, 62.5/125,
100/140, and 200/380l.mu.. The cost of multimode fibers in these
standard size ranges continues to decrease thereby making their use
in the present invention advantageous. A 1 km length of these
fibers, a length which may be necessary in large installations,
would present only a 2-6 dB loss.
The receiver 14 in FIG. 1 comprises demultiplexing wavelength
dispersive means 26, detectors 28, signal conditioning circuitry
30, and receiving unit 32. The demultiplexing wavelength dispersive
means 26 may comprise the same or similar type of wavelength
dispersive means used for the multiplexing wavelength dispersive
means 22 in the transmitter 12. Demultiplexing wavelength
dispersive means 26, however, obviously has one multimode fiber as
its input and three multimode fibers at its output (or a number of
output multimode fibers corresponding to the number of channels in
the system).
The output of the demultiplexing wavelength dispersive means 26 is
connected to detectors 28 which detect the various wavelength light
signal channels and convert them to electrical impulses
corresponding to standard logic levels at the transmitter 12.
Standard photodetectors may be used for this purpose, and
advantageously, these photo detectors may be lower speed detectors
which further reduce the cost of implementing the present
invention.
The preferred photodetector is a PIN diode which is small, fast,
and inexpensive. For cases where power levels at the detector are
very low (nW-pW range) avalanche photodiodes (APD) can be used. The
draw back of an APD is its high voltage (.about.100 v) requirement
and high temperature sensitivity compared to PIN diodes. The
temperature sensitivity, however, can be drastically reduced by
electronic feedback compensation. Both PIN diodes and APDs are
compatible with optical fibers in terms. of size (area) and
numerical aperture.
This is in contradistinction to the high speed photo detectors
needed in a TDM system as discussed above. For example, in order to
transmit 100 MHz aggregate bandwidth, the TDM case requires high
speed detectors with 100 MHz bandwidths, while in the 5-wavelength
WDM case, 20 MHz lower speed detectors are sufficient. Inexpensive
silicon can be used for the 800 nm window. This points up the
unexpected advantages of the present combination wherein the
components may be standard low cost components, which when combined
nonetheless yield very high aggregate bandwidth and low loss and
excellent power budget (approximately 20 dB power margin), high
tolerance to wavelength shift (approximately 10 nm), low cross-talk
(less than -20 dB optical or equivalently -40 dB electrical), and
full transparency (i.e., each wavelength carrier is transmitted
completely independently of other wavelength carriers).
The output of the photo detectors 28 is input to the signal
conditioning circuitry 30 which prepares the signal (at the same
logic levels 0-5 V digital or 1 V p-p analog as at the transmitter)
for input to the receiver 32 which may comprise any number of
receiving units such as standard data feed terminals.
Referring now to FIG. 2 a more detailed schematic of the exemplary
multiwavelength data communication link 10 in accordance with the
present invention with respect particularly to video signals is
depicted. A preferred transmitter 34 comprises multichannel video
source signal 36 having one output line 38 for each channel which
inputs the signal to the signal conditioning circuitry 40 through
75-ohm BNC connectors 42. Signal conditioning circuitry 40 may
comprise individual signal conditioning circuitry for each channel.
Signal conditioning circuitry 40 outputs the respective channels
electrically to standard laser drivers 42. Laser drivers 42 output
their electrical signal to lasers 44, there being one laser per
wavelength channel. The 3 mW lasers 44 output their laser light
waves onto fibers 48 through laser pigtails 46. Alternatively, 1 mw
ELEDS may be used. Fiber optic cables 48 are input to multiplexing
WDM 50. Pigtails 46 are essential to minimizing the effects of
feedback which include mode hopping between the lasers'
longitudinal modes, bias point shift, and wavelength shift due to
unwanted secondary resonator structure in the fiber. Without
control of feedback, bias point optimization at set-up will likely
be lost.
WDM 50 in a preferred embodiment may comprise the structure similar
to the WDM disclosed in U.S. Pat. No. 4,926,412. Briefly, the WDM
comprises a housing, a Fourier transform lens and a holographic
dispersion grating as shown in that patent. The WDM has close
location of the fibers about its optical axis and a holographic
dispersion grating which reduces losses in the system, the
essentials of which are incorporated herein as shown in FIG. 3
where d is the fiber core diameter, b is the core-core distance,
.varies..sub.0 is the Littrow angle, .LAMBDA..sub.// is the grating
period and f' is the focal length in medium n. (Alternatively, the
medium with refractive index n can be replaced by free space. Then,
only the lens remains with refractive index n, and f'=f). Unlike
state of the art systems in which only core diameter is critical,
cladding diameter as well as core diameter is important in the
present invention because cladding diameter determines the spacing
of the fibers. The use of highly-efficient holographic dispersion
gratings enables the multiplexing WDM to highly efficiently
multiplex the three or more channels simultaneously. It should be
noted that the grating may be transparent in a certain spectrum of
interest to additional wavelength channels. In such a case, the
optical wave of the additional channels may be focused to a fiber
on the right side of the grating to pass that optical wave through
the WDM unchanged. In FIG. 3, the input fiber is placed at the
center, slightly below the output fibers. In alternative
architecture, the input fiber can be placed with the output fibers
as in FIG. 2.
The output of the WDM 50 in FIG. 2 is preferably placed on a
multimode fiber having a core to cladding diameter ratio of
62.5/125, 100/140 or 200/380 standard in the industry. Each of the
signals in the multimode fiber 52 comprises a separate wavelength
in the fiber 52 (.lambda..sub.1 +.lambda..sub.2 +.lambda..sub.3) so
that the signals are multiplexed without interference, i.e., low
cross talk, and full transparency. Completely different signals can
be transmitted over the same multi-mode fiber. For instance, analog
video, and digital Ethernet signals can be simultaneously
transmitted on a single fiber unlike TDM systems. In other words,
due to full transparency, the multimode fiber 52 allows the
multiple channels space multiplexed therein to travel in the fiber
and behave as if the other channels traveling in the fiber did not
exist. Of course, each wavelength can, in addition, combine a
number of TDM channels. In addition, coupler 54 couples the 100/140
cable and the 200/380 cable if two different fiber sizes are used
(otherwise, the coupler 54 is not needed). The coupler 60 couples
the 200/380 fiber to a 100/140 fiber which is then input to a
demultiplexing WDDM 62 which may preferably comprise the same
components as the multiplexing WDM 50 in transmitter 34.
A preferred arrangement is the use of one size fibers to keep the
numerical apertures of the two fibers the same. Generally mixing
fiber sizes is undesirable because it increases power budget
(decreases power margin) and causes signal disturbance. These
effects are due to incompatible numerical apertures (NA) (which
defines the total internal reflection (TIR) angle of the fiber)
which causes the non-filling of all possible TIR angles within one
or both of the fibers. When a fiber whose full complement of TIR
angles is not filled is bent, the signal within the fiber is
disturbed. Modal partition noise due to differential attenuation of
optical paths taken in the multi-mode fiber can limit the "noise"
floor of the system when a coherent source such as an LD is used.
Thus, it is preferred to use fibers of equal diameter, especially
for analog systems.
On the other hand, in the unidirectional case, different sizes of
internal WDM fibers can be used. For example, in FIG. 2, where the
beam is transmitted only from the left to the right, fibers 48 can
have smaller cores than simple WDM fiber 52, even if the connector
54 is not used, and fibers 52 and 50 are identical. Analogously,
identical fibers 61', 61'', 61''', can have larger sizes than
external WDM fiber 52. In such a case, power subject and cross-talk
can be improved, but the system can be only unidirectional. In
order to preserve bidirectionality, however, all fibers may
preferably be identical.
Retrofit of existing systems frequently requires matching different
diameter fibers, however, therefore the arrangement in FIG. 2 is
apt. The WDM 62, however, instead of having three input fibers and
one multiplexed output fiber instead has one multiplexed input
fiber and three demultiplexed output fibers. The output of the WDM
62 is to photodetectors 64, there being one photodetector per
channel or fiber. At the input of each of the photodetectors 64 is
a photodetector pigtail each of which presents roughly a 1 dB loss
to the system. At the detector 64, the system again becomes
electrical and the output from the detector 64 is an electrical
signal to the signal conditioning circuitry 66 which prepares the
signal in standard manner for input into the video monitor 70 via
75-ohm BNC connectors 68. DIN connectors of course may be employed
as well.
RGB, CCTV, CAD Systems
This type of system may be used for a number of applications
including red-green-blue (RGB) color systems, CCTV, and computer
aided design (CAD). The modulation technique for RGB video may
preferably be analog whereas another preferred modulation technique
for CAD is digital. The present invention is capable of handling
both simultaneously as well. The number of channels that may be
implemented in the system depicted in FIG. 2 may be up to six
channels, based on present technology. In other words six different
wavelength carriers for carrying the information from each of the
respective channels may be employed. However, the improvement of
wavelength-shift control of LDs, and improvement of holographic
grating technology, may easily increase the potential number of
wavelength channels up to 20. The bandwidth of the RGB and CCTV
systems is in the range 10-30 MHz and for CAD is in the range
50-100 MB/s. Typical RGB or CCTV may be found in financial trading
firms for real time market data transfer, security systems
employing cameras, and information systems. CAD based systems, for
example, would be used extensively in the aerospace industry and in
university campuses.
Power Budget
The system described in FIG. 2 has very good power budget. The
basic components contributing to loss present in the system are the
laser pigtails 46 (.apprxeq.1 dB), photo detector pigtails 63
(.apprxeq.1 dB), multimode fiber 56 (.apprxeq.6 dB/km), and the two
WDMs (.apprxeq.3 dB each). In this embodiment, the typical total
power loss of these elements will be 10-15 dB. This yields
approximately a 20-25 dB power margin. Taking into account
wavelength shift, discussed in detail below, losses still remain
between 17-22 dB. Additional connecting losses due to the couplers
between the 100/140 and 200/380 multimode fibers are 11 dB but
total power margin still remains above 10 dB which is unexpectedly
high for this type of system.
An LD's optical intensity versus current and modulation depth, is
depicted in FIG. 4. Power budget is calculated by subtracting (in
dB) sensitivity of the detector from power of the source. Power
margin is power budget after power losses are subtracted. But these
calculations only represent DC budget while AC is of primary
interest because of finite source rise times. If a source is too
slow it is unable to raise system power to the maximum available.
Thus, AC power budget might be less than DC budget and should be
taken into account, as illustrated in FIG. 4.
Another limiting factor is dispersion. Two entirely different types
of dispersion are discussed with respect to the present invention.
Grating dispersion determines the WDM system's insertion loss and
cross-talk according to wavelength linewidth shift. Fiber
dispersion, on the other hand, limits signal speed. Light waves
traveling in a multi-mode fiber travel at different angles within
the fiber which causes them to travel at different speeds and thus
disperse. This is called multi-mode dispersion, and is of real
concern above 100 MHz, for fiber lengths of the order of a few
kilometers.
Input power to the laser pigtail is approximately 2 mW, to the
multiplexing fiber is 1 mW, to the multimode transmission line is
900 .mu.W, to the demultiplexing fiber is 250 .mu.W, to the
detector pigtail is 125 .mu.W, and to the detector is 120 .mu.W.
When calculating detected power at the photodetector, it must be
realized that this is a continuous intensity and produces only a
D.C. voltage, i.e., nonvarying electrical output. By using the
present invention, the efficiency of the system allows the depth of
modulation to be very small. The amount of detected modulation
intensity is approximately 1 .mu.W for a shot raise limited
system.
Real Time Market Analysis System
In use, as shown in FIG. 5, the system described may be used in a
real-time market analysis operation by plugging a module 70 (an APC
board which may be compatible with Europackaging standards)
containing the transmitter component circuitry 72, LDs 74, and WDM
76, into slot 80 (+12 v) of standard electronics rack 82 of the
trading computer electronics. The video signal is sent from the
transmitter via multimode fiber 84 between potentially many floors
in the trading offices and received by the receiver in rack 86
containing slot 87, electronics 88, detectors 90, and WDDM 92,
converted into electrical signals and then inputted to a video
monitor trading display 94. State of the art techniques send the
same signal either through a TDM system which has significant
transparency problems or through three fibers with obvious space
and cost constraints. Of course, the video inputs and outputs of
the system are standard base-band and comply with the RS170
standard.
Integrated Services Digital Network
FIG. 6A illustrates an ISDN implementation of the present
invention. The three channel system has dedicated channels for
voice, video and a computer network. Multiplexing link 96 contains
inputs for voice signals 98, video signals 100, and computer work
stations 102 connected by, for example, token ring. The signals are
fed into link 96, converted into light waves and multiplexed onto a
single multimode fiber bridge 104 and fed to demultiplexing link
106. Link 106 demultiplexes the light signals and converts each to
its respective electrical signal and then outputs them at output
108 for voice, 110 for video and 112 for computer networking. This
WDM bridge may be redundant with a state-of-the-art
single-wavelength bridge and both bridges may be connected
parallely and activated alternatively by a suitable switching
system.
FIG. 6B shows a remote computer workstation arrangement achieved by
use of the present invention. Current methods require the use of 4
fibers between the monitor/keyboard and the workstation. The system
in FIG. 6B comprises workstation 116, signal conditioning
electronics 118, laser diodes 120, photodetector 122, 3 dB power
splitter 124, WDM 126 having fiber array 128, lens 130, and grating
132. The work station is connected via the transmission fiber 134
to a WDDM 136 having fiber array 138, lens 140 and grating 142.
WDDM 136 is connected to multiple photodetectors 144 and laser
diode 146 which in turn are connected to electronics 148 to which
are connected keyboard 150 and monitor 152. The lines designated
RGB on workstation 116 are for transmission of data from
workstation 116 to the user at monitor 152 and keyboard 150. The
two lines marked DATA are for bidirectional data transfer at a
digital rate of 19.2 kB/s. The RGB lines are run at 35-50 MHz. By
using the arrangement in FIG. 6B only one fiber is needed to place
a workstation at a remote location greatly reducing costs by
eliminating four fibers currently used along with their associated
connectors and packaging.
It can be seen that WDM affords an additional degree of freedom in
the invention. In the context of space multiplexing, WDM is
competitive with current systems in the sense that fewer fibers are
required but complementary in the sense that each fiber may carry
more than one signal and with greater total bandwidth. In the
context of TDM, the WDM link is competitive in the sense that WDM
is able to handle more than one channel but complementary in that
TDM can be used in a WDM system to further increase system
flexibility.
Contrary to a TDM system which requires high speed sources, high
speed detectors, and different signals of different channels to be
electrically multiplexed, thereby making full transparency
unachievable, the present invention can transfer 2-6 channels of
information, comprising both analog and digital signals with nearly
full transparency. A number of different formats may be employed
for 2-6 channel operation, or more if the state of the art of light
sources improves. As shown in FIGS. 7A-D the present invention can
be employed as a 2-6 channel unidirectional system (FIG. 7A), where
T is a transmitter, R denotes its receiver, and .lambda..sub.n the
wavelength of each, a 2 channel bidirectional system (FIG. 7B), a 3
channel bidirectional system (FIG. 7C) and a 4 channel
bidirectional system (7D) as well as other combinations of these
architectures. This flexibility of design and implementation is due
to the physical independence of different channels in the present
invention due to the different wavelengths that are used to
transfer the information (.lambda..sub.n) and the highly efficient
manipulation of those light signals by the reflection WDM grating
having broad uniform wavelength characteristics, and a WDM system
based on paraxial optics transmission geometry. FIG. 8 depicts
wavelength separation of .lambda..sub.1 -.lambda..sub.4 in FIG. 7D
for instance. Wavelength separation between channels traveling in
the same direction must typically be greater than that for two
channels traveling in opposite directions to avoid cross-talk. Low
cost and standard fiber optic and light source components such as
CD LDs and standard multimode fibers may be used.
Tolerance to Wavelength Shift and Optimization
In order to better understand the merits of the combination of the
present invention and the invention's tolerance to loss of WDM
optical efficiency due to wavelength shift of LDs, the trade off
between optical losses and cross-talk is discussed. In FIG. 9,
wavelength shift d.lambda., source linewidth .delta..lambda., and
wavelength separation .DELTA..lambda. are depicted. .DELTA..lambda.
is the separation between the center Wavelenqths of the LDs (or
ELEDs), .delta..lambda. is the width of the spectrum of a LD or
ELED, and is typically greater for LEDs than for LDs, which
typically necessitate a greater .DELTA..lambda. for LED systems
(and will be assumed to be zero for the LD case here). d.lambda. is
the variation in the center wavelength of the LD or LED and is
typically greater for LDs than for LEDs or ELEDs (and will be
assumed to be zero for the ELED case below). Subsequently, the
adverse effects on performance in the case of ELEDs, due to a non
zero .delta..lambda. (line width) is discussed where d.lambda.
(shift) is ignored because LEDs have little wavelength shift. For
complete and accurate analysis one may assume that both
.delta..lambda. and d.lambda. are non-zero in the same model even
though they are secondary effects. For sake of analytic clarity of
evaluation, that is not done here.
There are a number of factors creating LD wavelength shift. Each of
these is discussed in detail because they are critical to
wavelength sensitive fiber optic WDM systems. For sake of
simplicity it is assumed here that the LDs are almost single
longitudinal mode, that is, typical 1-3 nm LD linewidths are
ignored, i.e. .delta..lambda.=0. Furthermore, a 2 dB WDM loss (for
zero wavelength shift) is assumed and only for a WDM employing a
volume holographic grating with high (greater than 90%) diffraction
efficiency within the spectrum of interest. In other cases, this
loss will be higher (say, 5 dB) but what is essential is the
substantial uniformity of the grating's wavelength characteristics
in the spectrum of interest. The assumption that the LD is single
mode is proper even though an LD linewidth can be 1 nm or slightly
more since a number of longitudinal modes are excited. In the
background discussion above, wavelength shift was assumed to be
zero as in the article, B. Moslehi, et al., "Fiber Optic Wavelength
Division Multiplexing Using Volume Holographic Gratings," 14 Optics
Lett. 1088 (1989) incorporated by reference herein. For wavelength
sensitive information transfer, however, such as in RGB or other
video, it is necessary to consider a non-zero d.lambda.. The link
of the present invention is able to minimize the effects of
wavelength shift thus offering unexpectedly high performance even
in the presence of a shifting wavelength source.
Thus, we assume for now that .delta..lambda.=0 and we proceed to
define the wavelength shift parameter: ##EQU1## Then, the optical
efficiency loss due to wavelength shift is ##EQU2## and
.zeta..sub.D =0, otherwise; or, in decimal logarithmic units,
Equation 2 has been derived based on a 1 dimensional model. For
more precise calculations, however, this formula should be replaced
by the following exact formula, based on the 2D model (see, for
example, J. W. Goodman, Introduction to Fourier Optics,
McGrew-Hill, 1975; Section 3): ##EQU3## and .zeta..sub.D =0,
otherwise; where k.sub.o =d/b.
Assuming that the fibers are packed closely together, b is also the
cladding diameter. It can be seen that both core and cladding
diameters are critical. This is because the jacket of the fiber is
preferably stripped to allow the fibers to be placed together as
closely as possible to achieve paraxiality and minimize dispersion
loss. Thus the cladding diameter is the determinative factor with
respect to paraxiality and dispersion loss.
For typical fibers, the core/cladding ratio is: 50/125.mu.,
62.5/125 and 100/140, 200/380. Thus, usually, ##EQU4## Eq. 2 then
takes the form
Eq. 2 is illustrated in FIG. 10, while Eq. 5, for
.DELTA..lambda.=30 nm, is illustrated in FIG. 11. FIGS. 10 and 11
depict the total loss of the optical signal for d.lambda..ltoreq.15
nm due to misalignment of the signal beam and the output fiber as
shown in FIGS. 12A and FIG. 12A shows partial misalignment
(.zeta..sub.D >0) and FIG. 12B shows total misalignment
(.zeta..sub.D =0). The results are also illustrated in Table 1 for
##EQU5##
__________________________________________________________________________
d.lambda. 1 nm 2 nm 3 nm 4 nm 5 nm 6 nm 7 nm 8 nm 9 nm 10 nm
__________________________________________________________________________
k 0.03 0.06 0.1 0.13 0.15 0.2 0.23 0.27 0.3 0.33 .xi. 93% 86% 80%
73% 67% 60% 53% 47% 40% 33% L.sub.D [dB] 0.3 0.65 1.0 1.36 1.73 2.2
2.75 3.27 4.0 4.8 L.sub.T [dB] 2.3 2.65 3.0 3.36 3.73 4.2 4.75 5.27
6.0 6.8
__________________________________________________________________________
The total loss, L.sub.T is calculated from the formula:
where 2 dB represents a rough estimation of additional losses such
as Fresnel loss, aberration loss, diffraction inefficiencies,
etc.
Formulation and discussion of a model of cross-talk is necessary to
fully analyze the merits of the claimed data link. Using the 1D
rectangular model as shown in FIG. 13 we have ##EQU6## To avoid
cross-talk then it is necessary that
Rewriting Eq. 7 in the form ##EQU7## and setting g=1, we obtain
##EQU8## And, to avoid cross talk, ##EQU9## The minimum loss,
L.sub.D min. occurs for b/d=(b/d).sub.c. Then, cross talk is at the
edge of acceptability, and ##EQU10## Equation 12 is illustrated in
Table 2 set out here.
TABLE 2
__________________________________________________________________________
.DELTA..lambda. 30 nm 30 nm 30 nm 30 nm 30 nm 30 nm 30 nm 30 nm 30
nm 30 nm d.lambda. 1 nm 2 nm 3 nm 4 nm 5 nm 6 nm 7 nm 8 nm 9 nm 10
nm k 0.03 0.06 0.1 0.13 0.15 0.2 0.23 0.27 0.3 0.33 ##STR1## 1.03
1.06 1.11 1.15 1.18 1.25 1.3 1.4 1.43 1.5 (.zeta..sub.D)min 97% 94%
88% 85% 82% 75% 70% 62% 57% 50% (L.sub.D)min 0.1 0.2 0.5 0.7 0.86
.12 1.5 2.0 2.4 3.0 [dB] (L.sub.T)min 2.1 2.2 2.5 2.7 2.86 3.2 3.5
4.0 4.4 5.0 [dB]
__________________________________________________________________________
From Table 2 it can be seen that the minimum loss, (L.sub.D)min, is
quite low, and total loss, (L.sub.T)min, is lower than 5 dB, even
for wavelength shift of d.lambda.=10 nm. Now, comparing four fibers
of the present invention from the point of view of Eq. 10, and
assuming that ##EQU11## we find the critical k.sub.c value from the
relation ##EQU12## and
It can be seen that for
cross talk can be ignored. (L.sub.D).sub.c and (L.sub.T).sub.c for
(d.lambda.)=(d.lambda.).sub.c can be calculated and illustrated as
in Table 3 below.
TABLE 3 ______________________________________ Fibers ##STR2##
##STR3## k (d.lambda.) .zeta..sub.D L.sub.D
______________________________________ A 200/380 1.9 0.47 14.1 nm
10% 10 dB B 62.5/125 2 0.5 15 nm N/A .infin. C 50/125 2.5 0.6 18 nm
N/A .infin. D 100/140 1.4 0.28 8.4 nm 60% 2.2 dB
______________________________________
From Table 3 it can be seen that for fibers 62.5/125 and 50/125,
cross-talk is not a limitation and very high wavelength shifts
(greater than 10 nm) can be tolerated assuming that loss is
acceptable. For 200/380 fiber, however, wavelength shift must be
less than 14 nm in order to avoid significant cross talk, and for
100/140 fiber, wavelength shift must be less than 8 nm in order to
avoid cross talk. In summary, for 200/380 fiber and for
d.lambda.=14 nm, L.sub.D =10 dB and L.sub.T =12 dB without cross
talk. For 100/140 fiber and for d.lambda.=8 nm, L.sub.D =2 dB and
L.sub.T =4 dB. It can be seen from the above that the
multiwavelength data communication link of the present invention
can achieve high .zeta..sub.D and low cross-talk even for high
wavelength shifts assuming the optimized b/d ratio, close to
(b/d).sub.c, given in Tables 2 and 3 is followed.
For a given wavelength shift, the b/d ratio should be slightly
higher than the (b/d).sub.c ratio and therefore the fiber
parameters may be adjusted for wavelength shift. This is
illustrated by Eq. 16. For a given k (i.e., wavelength separation,
.DELTA..lambda., and wavelength shift, d.lambda.), the critical
geometric ratio of the fiber parameters (d/b).sub.c can be found
and from these parameters the k.sub.c parameter can be found. Using
the value of the k.sub.c parameter and .DELTA..lambda. the critical
(d.lambda.).sub.c can be found. According to Eq. 16, if wavelength
shift is smaller than (d.lambda.).sub.c significant cross-talk can
be avoided.
For example, referring again to Table 2, for .DELTA..lambda.=30 nm
and d.lambda.=8 nm, k=0.27 and (b/d).sub.c =1.4. It then follows
that for any fibers with b/d>(b/d).sub.c, and for
d.lambda..ltoreq.8 nm cross-talk is avoided. If the fiber of
interest has the required critical parameters, then its loss will
be minimal (L.sub.D min=2.4 dB, and L.sub.T min=4.4 dB) and
significant cross-talk may still be avoided even for wavelength
shift as high as d.lambda.=8 nm.
Eq. 14 has been illustrated in FIG. 14. It is seen that the actual
fiber value of b/d determines k.sub.c and that for k<k.sub.c we
have low cross-talk, while for k>k.sub.c cross-talk is high.
k=k.sub.c is the optimum case from the point of view of
minimization of cross-talk and insertion loss due to wavelength
shift, and misalignment.
Using Eq. 12, we obtain ##EQU13## This equation allows calculation
of acceptable wavelength shift, for predetermined insertion loss.
For example, for .zeta..sub.D =0.5, and L.sub.D =3 dB (equivalent
to L.sub.T =5 dB, according to Eq. 6), we obtain k.sub.o =0.33,
which, for .DELTA..lambda.=30 nm, gives (.delta..lambda.).sub.o
=k.sub.o .DELTA..lambda.=10 nm. On the other hand, combining Eqs.
10, and 17, We obtain ##EQU14## illustrated in FIG. 15. It is seen
that, for .zeta..sub.D =0.5 (and L.sub.D =3 dB), (b/d).sub.c =15.
It should be emphasized that these results are only approximate
because the 1 D rectangular cross-talk model, illustrated in FIG.
13 is used.
The following example illustrates the optimization principle,
discussed above. Assuming, that WDM dispersion loss of 3 dB is
acceptable, the fiber should have b/d ratio of 1.5, close to that
of 100/140.mu. - fiber (see Table 3). If, however, the actual fiber
will have b/d=2, such as 62.5/125 fiber, the dispersion loss will
be higher, since L.sub.D =5 dB. This optimization principle is
illustrated in Table 4, where optimized valued of L.sub.D, k,
(d.lambda.), and (b/d).sub.c, are compared in a modified version of
Table 2.
TABLE 4 ______________________________________ .DELTA..lambda. 30
nm 30 nm 30 nm 30 nm 30 nm 30 30 nm nm .zeta..sub.D 0.8 0.7 0.6 0.5
0.4 0.3 0.2 L.sub.D 1 dB 1.5 dB 2.2 dB 3 dB 4 dB 5 dB 7 dB k 0.17
0.23 0.28 0.33 0.37 0.41 0.44 (d.lambda.) 5 nm 7 nm 9 nm 10 nm 11
nm 12 13 nm nm ##STR4## 1.2 1.3 1.4 1.5 1.6 1.7 1.8
______________________________________
According to Table 4, we can either adjust the fiber's b/d ratio,
to a predetermined maximum wavelength shift, or select acceptable
wavelength shift for the actual fiber. For example, for 100/140
fiber, we have b/d=1.4, and according to Table 4, the maximum
wavelength-shift, in order to minimize dispersion loss, is 8.6 nm,
for L.sub.D =2.2 dB, and L.sub.T =4.2 dB. Of course, the optimum
b/d - ratio, can always be adjusted either by removing part of the
cladding (if b/d>(b/d).sub.c), or using spacer elements if
b/d<(b/d).sub.c.
The presented optimization procedure determines the optimum WDM
design, according to given LDs and system geometrical tolerance,
defined by maximum wavelength shift, d.lambda..sub.o, and optimum
fiber b/d - ratio, defined by (b/d).sub.c, according to Tables 1-4,
and Eqs. 1-18. Since, the WDM grating and WDM lens are also
optimized, this procedure optimizes all basic components of the WDM
system, from the point of view of dispersion loss and cross-talk
minimization.
Eqs. 10, 17, and 18 are the basis for an optimization formula that
allows for the minimization of loss, L.sub.D, for a given
wavelength shift. The multiwavelength data communication link of
the present invention may preferably be designed according to the
following:
1. Assuming that the system has wavelength shift not larger than
d.lambda.=d.lambda..sub.c, the coefficient k=k.sub.o is calculated
for a given d.lambda.;
2. Using Eq. 10 the optimum fiber geometry (b/d).sub.c is found.
Then, for b/d=(b/d).sub.c, the system will have the minimum loss
(L.sub.D)min within wavelength shifts
d.lambda..ltoreq.d.lambda..sub.c still preserving low cross
talk;
3. If b/d>(b/d).sub.c, then the system has higher than minimum
loss (L.sub.D)>(L.sub.D)min, within wavelength shifts
d.lambda..ltoreq.d.lambda..sub.c.
Of course, wavelength shift cannot be totally eliminated because it
is caused by many changing factors such as temperature, aging,
fiber geometry tolerance, current modulation, source pigtailing
feedback, and many others. The above optimization, however, may
minimize wavelength shift in the system of the present invention
providing unexpected superior performance. Furthermore the
condition d.lambda.=0 occurs where the optimum alignment of the
fiber array, lens, and grating are in paraxial alignment as shown
in FIG. 3 and, in a preferred embodiment, as shown in U.S. Pat. No.
4,926,412 and the reference B. Moslehi, et al., Optics Letters, 14,
1088 (1989). The multiwavelength data communication link of the
present invention is able to minimize the effects of wavelength
shift because of the particular combination of mature CD LD
technology, within the single transmission window 750 nm to 850 nm,
the use of standard multimode fibers, and the paraxial structure of
the WDM including the fiber grouping, lens, and grating, be it
holographic, including nonuniform volume holographic gratings, or a
photoresist grating. It should be noted that the optimization
procedure, determined by Eqs. 1-18, is independent of absolute LD
wavelength values.
Single Mode Fibers and DFB Lasers
Equations 1-18 can also be used to analyze single-mode fiber
applications. Typical cladding/core ratios for single mode fibers,
however, are rather high. For example, for 9/125.mu.-single-mode
fiber, ##EQU15## Equation 2 thus takes the form
and only small k-values are acceptable to avoid high insertion
loss. Fortunately, DFB (distributed feedback) lasers, typically
used in single mode applications, have highly stable wavelengths;
thus, DFB k-ratios may be very low, even for small wavelength
channel separation, .DELTA..lambda.. For example, for a DFB
wavelength stability of 1 GHz, d.lambda..perspectiveto.0.05 .ANG.,
and for .DELTA..lambda.=1 nm, k=0.005. Using Eq. 20, we obtain
.zeta..sub.D =0.93, and L.sub.D =3 dB. In such a case, cross-talk
is no problem, since, according to Eq. 14, b/d>>(b/d).sub.c.
Of course, if a part of the cladding is removed, then b/d<14,
and Eq. 19 should be modified accordingly.
Of much more serious concern is fiber array tolerance. Eq. 2 may be
generalized to the form, ##EQU16## where k.sub..lambda. represents
wavelength shift (in Eq. 2, k=k.sub..lambda.), and K.sub.x
represents the fiber array geometrical tolerance in the form
##EQU17## where dx is the fiber core center misalignment versus the
ideal x-location. Setting k.sub.x =k.sub..lambda., we can find the
equivalent optical frequency and wavelength shifts, illustrated in
Table 5, for .DELTA..lambda.=1 nm, and b=125.mu..
TABLE 5 ______________________________________ dv 1 GHz 2 GHz 3 GHz
5 GHz 8 GHz 10 GHz ______________________________________ d.lambda.
0.05 .ANG. 0.1 .ANG. 0.15 .ANG. 0.25 .ANG. 0.4 .ANG. 0.5 .ANG. dx
0.6.mu. 1.2.mu. 1.8.mu. 3.mu. 4.8.mu. 6.mu.
______________________________________
According to Table 5, fiber misalignment is usually the dominating
factor for typical DFB lasers; thus, ##EQU18## For example, for
9/125.mu. fiber b/d.perspectiveto.14, and d.sub.x =2.mu., k.sub.x
=0.016, and, according to Eq. 23, .zeta..sub.D =0.78, or L.sub.D
=1.1 dB. Therefore, using the design of the present invention, the
single-mode WDM link is achievable but only using DFB lasers and
few-micron fiber-array tolerance and present day technology.
Another way to minimize k.sub.x would be to tune the DFB lasers'
wavelengths; then wavelength separation would be non-uniformly
distributed.
The other fundamental advantages of the present invention for
single-mode fiber WDM is that by using Littrow volume holographic
gratings, the angle between diffracted and incident beams is close
to 180.degree.. Therefore, according to the theory of volume
holograms (see H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969),
the coupling constants, .nu., for both polarizations, TE and TM are
identical:
and the WDM system is polarization-insensitive. Therefore, using
parallel-oriented polarization-preserving single-mode fibers, and
Eq. 24, the single-mode WDM system of the present invention
preserves polarization.
Still another advantage of the single-mode WDM system of the
present invention is the fact that Littrow volume holographic
gratings have uniform wavelength characteristics within a very
broad spectrum (>100 nm). Therefore, assuming .DELTA..lambda.=1
nm, large numbers of wavelength channels, N may be employed.
However, assuming N=20, and b=125.mu., we obtain the size of the
fiber array: N.multidot.b=2.5 mm, and in order to preserve the
paraxiality condition, the focal length of the WDM system, f,
should satisfy the following relation.
Therefore, the sizes of the WDM system (both single-mode and
multi-mode) are determined by the number of channels. Of course,
the maximum number of wavelength channels is also limited by the
acceptable signal dispersion developed under long-length fiber
propagation. For example, if single-mode fiber dispersion is 2
psec/1 nm.multidot.1 km, and the number of wavelength channels is
N=20, for fiber length L=20 km; then to achieve channel separation
of .DELTA..lambda.=1 nm, we obtain a total signal dispersion of 800
psec. This dispersion can be reduced by half if we replace
.DELTA..lambda.=1 nm, by .DELTA..lambda.=0.5 nm. But then,
according to Eq. 21, insertion loss doubles.
Therefore there is trade-off between the number of channels, N,
wavelength separation, .DELTA..lambda., signal dispersion, and
system size. The optimum design may achieve a balanced trade-off
among system requirements, say, maximum insertion loss of 3 dB, and
maximum signal dispersion of 1 nsec for 20 km - fiber length, which
in turn determines the maximum acceptable number of channels in the
system. In summary, Eqs. 1-25 permit optimization of a WDM system
design including: (1) fiber tolerance, dx; (2)DFB laser wavelength
tolerance, d.lambda.; (3) wavelength separation, .DELTA..lambda.;
(4) number of wavelength channels, N; (5) insertion loss,
.zeta..sub.D, L.sub.D, L.sub.T ; (6) cross-talk, determined by
(b/d).sub.c, (7) fiber length, L; and (8) signal dispersion, and
fiber cladding/core ratio, b/d, etc.
As an example of such a global design, we analyze below each aspect
of a fiber-optic WDM link design including: fiber geometry, WDM,
light sources, detectors, and signal parameters for digital
processing. The power budget calculation is very simple and
straight-forward for LD single-mode fiber optics. Assuming a
single-mode fiber (9/125 .mu.m) with 0.5 dB/km fiber loss, and 35
dB-power budget, we obtain, for typical 3 mw-LD, and typical
coupling and WDM (2 dB) losses, an excellent power margin of about
15 dB for 20 km-link. Signal dispersion, q, can be calculated from
the following formula
where .rho. is signal dispersion per 1 nm and unit length, and
q.sub.o is maximum acceptable signal dispersion. For example, for
.rho.=2 psec/1 nm.multidot.1 km, .DELTA..lambda.=1 nm, N=20, and
L=20 km, we obtain q=760 psec and the condition of q.sub.o =1 nsec,
which is the maximum acceptable rise time, is satisfied. However,
assuming q.sub.o =500 psec, we would need to decrease
.DELTA..lambda.; for .rho., N, L=constant. This, however, would
increase WDM insertion loss, L.sub.D, and cross-talk, C, according
to Eqs. 20 and 21.
In addition, .DELTA..lambda., also determines WDM geometry,
according to the basic formula
Since, in this equation, b is fixed by fiber geometry (here b=125
.mu.m) assuming no-space between fibers, the only variables are the
factors of the following product:
The focal length of the lens, f, however, cannot be too small
because the paraxial condition of Eq. 25 must be satisfied. For
example, for N=20 and b=125.mu., we obtain
in order to satisfy the paraxial condition,
The minimum lens diameter, D, is related to f, by the well-known
formula, ##EQU19## where NA is the fiber numerical aperture and f#
is the f-ratio. For NA=0.2 and f=3 cm, we obtain D=1.2 cm. Thus,
the total WDM sizes, (2f, D, D) are approximately
Using Eq. 27 for b=125.mu., f=3 cm, and .DELTA..lambda.=1 nm, we
obtain K.sub..lambda. =4.1.mu..sup.-1. Assuming .lambda..sub.L
=1300 nm, and using Eq. 44, .alpha.=77.degree. is obtained. In
order to decrease this angle, K.sub..lambda. must be decreased
which increases f because, according to Eq. 28, the product
f.multidot.K.sub..lambda. should remain constant. For example, by
increasing f three times, we obtain f=9 cm and instead of Eq. 32 we
have
This is still acceptable from a practical point of view (it should
be remembered that we consider here a very large number of
wavelength channels, N=2D). Then, from K.sub..lambda.
=1.36.mu..sup.-1 and from Eq. 44, .alpha.=46.degree..
Also from Eq. 43, 1/.LAMBDA..sub..vertline..vertline. =1000 l/mm,
and from .LAMBDA.=.lambda..sub.L /2n, we obtain .LAMBDA.=0.42.mu.m,
for n=1.55 (for DCG). In a single-window system, the grating O.D
can be low (e.g., O.D=1.5); but if the system has two windows (some
wavelengths are transmitted through the WDM) the grating O.D must
be high in order to preserve low cross-talk, C, according to Eq.
81, below). For example, for C=-50 dB, O.D=5.0. The grating
transmission, T, for the remaining part of the spectrum should be
high, say, T>90% (0.5 dB-loss).
Using Table 5 and assuming DFB-laser sources with moderate
frequency control, d.nu.=2 GHz, we obtain d.lambda.=0.1 .ANG. and
k.sub..lambda. =0.01, and assuming fiber tolerance d.sub.x =3.mu.,
we obtain k.sub.x =0.025; thus k=k.sub..lambda. +k.sub.x =0.035,
and from Eq. 31,
i.e., L.sub.D =3 dB and L.sub.T =5 dB. Also, from Eq. 14
(b/d).sub.c =1.03; and ##EQU20## since the actual (b/d)=14.
Therefore, system cross-talk is extremely low (<-50 dB); and
thus, BER<10.sup.-12 and signal data rate, B.sub.o, also can be
high (>1 Gbit/sec per channel). Thus, the aggregate system data
rate, B.sub.T, can be extremely high,
All of this data has been summarized in Table 6 for a single-mode
21-channel WDM link. For simplicity, the data has been categorized
within the following classes: fiber (F), WDM, light source (S),
grating (G), lens (L), signal dispersion (D), and general link
parameters (L-P).
TABLE 6 ______________________________________ No. Parameter Symbol
Category Value ______________________________________ 1 Fiber Core
Diameter d F 9 .mu.m (Single-Mode) 2 Fiber Cladding b F 125.mu.
Diameter 3 Source Power (DFB P.sub.i S 3 mw Laser) 4 Power Budget
L-P 35 dB 5 Fiber Loss F 0.5 dB/km 6 Link Length L F 20 km 7 Power
Margin L-P 15 dB 8 Acceptable Signal q D 1 nsec Dispersion 9 Number
of .lambda.-Channels N WDM 20 + 1* 10 Channel .lambda.-Separation
.DELTA..lambda. WDM 1 nm 11 Unit Signal Dispersion .rho. D 2 psec/
1 nm .multidot. 1 km 12 Total Signal Dis- q D 760 psec persion** 13
Grating Dispersion K.sub..lambda. G 1.3.mu..sup.-1 Coefficient 14
Lens Focal Length f L 9 cm 15 Fiber Numerical NA F 0.2 Aperature 16
Lens Diameter D L 3.6 cm 17 WDM Sizes WDM (18 cm, 3.6 cm, 3.6 cm)
18 Littrow Wavelength .lambda..sub.L WDM 1300 nm 19 Littrow Angle
.alpha. G 46.degree. 20 Grating Resolution 1/.LAMBDA..sub.// G 1000
l/mm 21 Grating Constant .LAMBDA. G 0.42 .mu.m 22 DCG Refractive
Index n G 1.55 23 Grating O.D. O.D. G 1.5 (Single-Window) 24
Grating O.D. O.D. G 5.0 (Dual-Window) 25 Grating Transmission T G
.gtoreq.90% (0.3.mu.- 3.0.mu.) 26 DFB Frequency d.nu. S 2 GHz
Control 27 DFB Wavelength d.lambda. S 0.1.ANG. Control 28 Fiber
Tolerance dx F .+-.3.mu. 29 Wavelength-Shift k.sub..lambda. S 0.01
Coefficient 30 Fiber Tolerance k.sub.x F 0.025 Coefficient 31
Global .lambda.-Coefficient k WDM 0.035 32 Dispersion Loss L.sub.D
WDM 3 dB 33 WDM Total Loss L.sub.T WDM 5 dB 34 Cross-Talk Critical
(b/d) ##STR5## WDM 1.03 35 WDM Cross-Talk C WDM <-50 dB 36 BER
BER L-P <10.sup.-12 37 Data Rate, Per Channel B.sub.o L-P 1
Gbit/sec 38 Data Rate, Aggregate B.sub.T L-P 21 Gbit/sec
______________________________________ *Assuming dualwindow WDM
(otherwise, N = 20) **For singlewindow signals
Edge Limiting LED Multiwavelength Link
A multiwavelength data communication link using an edge limiting
LED (ELED) is now discussed. In the previous discussion of LDs, it
was assumed that the LDs were single mode, i.e., .delta..lambda.=0.
In the discussion of the ELED case in which the line width of the
source cannot be ignored readily as in the LD case, we take into
account line widths of between 50-60 nm for ELEDs. Now, however,
wavelength shift, d.lambda., is assumed to be zero because LEDs in
general suffer very little from this effect.
The first use of ELEDs in the present invention is only slightly
different than the use of LDs, analyzed by Eqs. 1-18. In the ELED
case we replace Eq. 5, by the following approximate equation:
where, instead of Eq. 1 we have, ##EQU21## and wavelength
linewidth, .delta..lambda., is illustrated by FIG. 9 (here,
customary to LDs, d.lambda.=0). It should be noted that in the case
of ELEDs their extended spectrum should be integrated over
dispersion efficiency .zeta..sub.D (See Eq. 37) in order to obtain
aggregate coupling efficiency, Q, similar to Eq. 57. In such a
case, for the uniform part of the ELED spectrum, we obtain Q=0.5.
The equivalent of Eq. 10 has similar form, ##EQU22## Therefore, the
optimization analysis is similar to that for LDs, except d.lambda.
is replaced by .delta..lambda., and k by k'. Assuming L.sub.D =3
dB, we obtain from Eq. 37, k'=0.5, and, according to Eq. 39,
(b/d).sub.c =1.33. Now, however, the Wavelength separation,
.DELTA..lambda., must be made larger since for .delta..lambda.=40
nm, .DELTA..lambda.=80 nm. On the other hand, for (b/d).sub.c =2,
from Eq. 39 we obtain k'=1, still preserving Q=0.5, for the uniform
part of the ELED spectrum.
The second use of ELEDs in the present invention is such that the
WDM is used to filter out all but preferably the center frequency
of each of the ELEDs used as sources. The WDM may also be used to
filter out all but a frequency slightly off center of each of the
ELEDs with satisfactory results but with some power loss. This
filtering ability of the WDM is extremely advantageous because it
results in an effective .delta..lambda. that is smaller than
.delta..lambda. of the ELED itself. This sampling or slicing of LED
spectra is illustrated in FIG. 16A which shows the spectra for
three ELEDs filtered by the WDM leaving only the center frequency
for use in the data link.
It can be seen that for each of the LEDs .delta..lambda., the
region surrounding the central wavelength .lambda.1, .lambda.2, or
.lambda.3, is far smaller than the full, unfiltered spectrum of the
ELED. As seen in FIG. 16B, when the filtered central wavelengths of
each of the LEDs is multiplexed, the small .delta..lambda. of each
allows close juxtaposition of each of the carriers in wavelength.
In this example, .lambda.1, .lambda.2, and .lambda.3 may equal
1300, 1330, and 1360 nm. Therefore, the distance .DELTA..lambda.
between the central frequencies of each of the filter LED spectra
is on the order of about 30 nm rather than, in an unfiltered case,
hundreds of nm.
Filtering Effect
An analysis of the filtering effect of the ELED multiwavelength
data communication link of the present invention is now discussed.
There are basically two types of LEDs, surface emitting LEDs
(conventional) having wavelengths located typically at 800 nm and
at 1.3.mu., Lambertian type spectra, and broad line widths,
.delta..lambda., up to 200 nm. The second type of LED is the ELED
which has wavelengths at 1.3.mu. and narrower line widths of about
60 nm and recently down to 40 nm. ELEDs are not Lambertian in
nature, however. Therefore, coupling efficiency .zeta. of ELEDs is
much higher than conventional LEDs. Coupling efficiency .zeta. is
given by the following equation ##EQU23## where NA.sub.F is the
numerical aperture of a fiber connected to the LED (and d is its
core diameter) and the lower NA.sub.L is the numerical aperture of
the LED itself (and D' is its diameter). In the case where a LED is
used, NA for the LED is 1 because it is a Lambertian source.
Assuming a typical NA for a fiber is between 0.2 to 0.5, coupling
efficiency .zeta. turns out to be only about 25% (assuming d/D'
equals roughly 1). In the ELED case, NA<1 and therefore .zeta.
is much higher.
In the WDM used in the present invention, the source fiber emits
light with a divergence determined by the fiber's numerical
aperture NA, the light is collimated by the aspheric lens and
reflected and dispersed by the grating that is mounted at Littrow
angle for the central channel. The reflected light is reflected by
the same aspheric lens into one of the receiving fibers in the
fiber array determined by the wavelength of the light source.
The fundamental relation between the fiber spacing b, the channel
spectral separation .DELTA..lambda., and the focal length of the
lens f can be derived from the basic grating formula for the
first-order diffraction: ##EQU24## where .lambda. is the light
wavelength in vacuum, .LAMBDA..sub..vertline..vertline. is the
grating constant, .alpha. is the incident angle with respect to the
normal to the grating, and .beta. is the diffraction angle. At the
Littrow configuration, where .alpha.=.beta., the beam is reflected
directly backward and the angular distortion of the reflected beam
is minimized. If Eq. 40 is differentiated for the fixed wavelength
case the following is derived ##EQU25## where K.sub..alpha. is the
geometrical magnification coefficient (which indicates that the
system is a one-to-one imaging system) and ##EQU26## Thus, the
dispersion factor K.sub..lambda. is shown to be ##EQU27## Rewriting
Eq. 44 for .DELTA..beta. we get
and the fiber spacing b (center-to-center) is given by
Using the following equation ##EQU28## where d is the core
diameter, and substituting in Eq. 45 from above, the following is
obtained ##EQU29## and substituting in Eq. 42, the equation
defining k' is obtained: ##EQU30##
Since light emitted by a LED or ELED is polychromatic (k'>0)
some fraction of light energy will be lost in the fiber cladding.
Considering the LED beam as a continuous superposition of
monochromatic components, we can introduce the formula similar to
Eq. 2 in the form: ##EQU31## determines the angular sizes of the
fiber core and ##EQU32## where .delta..lambda..sub.a determines the
wavelength shift of a given monochromatic component. Substituting
Eqs. 51 and 52 into Eq. 50, we obtain ##EQU33## and .zeta..sub.D
=0, otherwise, where ##EQU34## Equations 50-54 are more precise
than Eqs. 37-39, since they include all spectral components, not
only boundary ones. According to Eq. 53, if k.sub.a =d/b, then
.zeta..sub.D =0. Therefore, the maximum wavelength shift, accepted
by the output fiber, is ##EQU35## and the total spectral linewidth,
accepted by the output fiber, is in accordance with Eq. 49. It
should be noted, however, that only a fraction of this spectrum
will be accepted by the fiber. The general formula for coupling
efficiency is ##EQU36## where .zeta..sub.D is determined by Eq. 53,
and G(.lambda.) determines the source power (usually symmetrical)
spectrum within linewidth .delta..lambda..sub.1 =2.delta..lambda.,
where
and .lambda..sub.o is the central wavelength of the spectrum.
Substituting Eq. 53 into Eq. 57 and assuming uniform spectral
distribution, we obtain ##EQU37## where .delta..lambda..sub.1
=2.delta..lambda..sub.a is the accepted spectral linewidth.
Substituting Eq. 56 into Eq. 59, we obtain
i.e., 3 dB dispersion coupling loss, independently on k.sub.1 (3 dB
of input energy is lost in fiber cladding). In order to avoid
cross-talk, k.sub.1 .ltoreq.1; thus according to Eq. 56,
b>2d.
Broad-Band Source
The WDM design presented above can also be used to obtain a highly
efficient broad-band source from a number of narrow-band sources,
slightly shifted in the .lambda.-domain. In such a case, FIG. 16A
can be interpreted in a different way, namely, only the three
cross-hatched spectral areas are present representing three
.lambda.-separated ELEDs, or other sources. (The remaining part of
the spectra does not exist). Assuming m-number of such sources, and
(b/d)=2, we obtain using similar analysis as before, that the
maximum coupling efficiency, Q, to the multiplexed fiber will be
50%, independently of m. To the contrary, for state-of-the-art
couplers, such efficiency will be (1/m), according to the
well-known brightness theorem (see, e.g., M. Born and E. Wolf,
Principle of Optics, Pergamon press (1970)). Therefore, the
coupling efficiency gain, due to the present invention, will be
especially visible for large m. For example, for m=10, we obtain
only Q=10% for the state-of-the-art couplers, versus Q=50%, for a
WDM system being subject of this invention.
WDM Insertion Loss
In the case of WDM demultiplexing, in order to calculate the
insertion loss, we need to use a formula similar to Eq. 57, and an
analogous formula for cross-talk. For sake of simplicity, however,
we will use the boundary formula similar to Eq. 7 in the form
##EQU38## or using Eq. 56, ##EQU39## Assuming a low cross-talk
condition, similar to Eq. 8,
and replacing in Eqs. 61-63, (b,d) by (b.sub.2, d.sub.2) we finally
obtain ##EQU40## and using Eq. 56, ##EQU41## Therefore, the
symmetric case is only for ##EQU42##
Comparing Eqs. 64 and 65 it is seen that the WDM system, (d.sub.1,
b.sub.1), performs spectrum sampling (k=2d.sub.1 /b.sub.1) while
the WD(D)M system (d.sub.2, b.sub.2) can effectively demultiplex
the sampled spectrum with low cross-talk, assuming Eq. 65 is
satisfied.
Equation 64 used separately, also demonstrates that we can use two
LEDs, only slightly separated, in order to preserve low cross-talk.
Using Eq. 64, we have ##EQU43## Thus, for b/d=2, we obtain k=1, and
.delta..lambda.=.DELTA..lambda.. Therefore, even the so-called
dual-wavelength approach with LEDs, can be effective in this case,
assuming grating wavelength characteristics are sufficiently broad
to cover both LED spectra.
WDDM Spectrum, Temperature, or Spectral Signature Measurement
Equation 56, used for the WDDM case (thus: d=d.sub.2, b=b.sub.2),
can be also applied to a novel design of a compact optical spectrum
analyzer (or stationary spectrophotometer) of the present
invention. In such a case, Eq. 56 can be rewritten in the following
form ##EQU44## where d.sub.2 determines either fiber core diameter
(or pixel diameter of fiber/photodector array). The difference
between the fiber array (see FIG. 3) and photodetector (CCD) area
is not a fundamental problem but only an engineering concern
because detector pixels can be either placed directly in the output
plane or connected through the fiber array. In a similar way, the
lens can be replaced by a collimating/concentrating mirror with the
profile close to an off-axial paraboloid. In any case, we have four
possible packaging geometries of detector arrays determined by Eq.
56A, combining .delta..lambda..sub.o wavelength resolution (or
wavelength range averaged within a single-pixel area) with
.DELTA..lambda. separation between the adjacent central pixel
wavelengths:
a) for b.sub.2 >2d.sub.2, then .delta..lambda..sub.o
<.DELTA..lambda., and some part of the wavelength spectrum is
not detected by the pixels at all;
b) for b.sub.2 =2d.sub.2, then .delta..lambda..sub.o
=.DELTA..lambda., and the spectral gaps are eliminated but
cross-talk remains low;
c) for d.sub.2 <b.sub.2 <2d.sub.2, then ##EQU45## and some
parts of the spectra overlap; and d) for b.sub.2 =d.sub.2, then
.delta..lambda..sub.0 =2.DELTA..lambda., and pixel packaging is the
closest possible.
Among those four cases, case b) seems preferable, but other cases
may also be applicable. Design of such a WDDM spectrophotometer is
purely based on the grating WDM being of the present invention, and
it can be illustrated by a table similar to Table 6, either for the
single-mode or multi-mode fiber case, including parameters (9, 10,
17, 18, 31, 32, 33, 34, 35) related to a WDM system. The WDDM
spectrophotometer of the present invention can use gratings similar
to the WDM fiber-optic link case while the WDM lens can be replaced
by a collimating mirror if the lens' chromatic aberration is too
large, or if using a lens is prevented for longer infrared (IR)
wavelengths of interest. Also, conventional fibers can be replaced
by special fibers transparent to longer IR wavelengths (5-10.mu.).
The design, however, remains fundamentally the same; thus
preserving low grating dispersion loss and low cross-talk due to
paraxial geometry of the WDDM system. Such a spectrophotometer
design can be particularly useful for a moderately broad spectra of
interest (<500 nm) and for limited spectral resolution. For
example, for b.sub.2 =2 d.sub.2 and for .delta..lambda..sub.o
=.DELTA..lambda.=0.5 nm, we can cover a 500 nm spectral range with
N=1000 pixels and 0.5 nm spectral resolution. Thus, we can receive,
parallely, 1000 wavelength readings using a relatively simple and
compact WDDM system. The obvious applications are Raman
spectroscopy, where Stoke's lines or anti Stokes can be measured,
usually in the visible and/or near IR 100- 500 nm spectral range.
In such a case, the Rayleigh spectrum may be blocked using a
holographic Raman filter such as is fully disclosed in U.S. patent
application Ser. No. 464,116, filed Jan. 12, 1990.
In an other related application, high-temperature electromagnetic
radiation can be measured using a WDDM spectral thermometer similar
to the WDDM spectrophotometer (or spectroradiometer) discussed
above. The typical temperature ranges are 500.degree.
C.-5000.degree. C., equivalent to peak wavelengths of radiation,
located in visible, near IR, and middle IR regions. In such an
application, the number of wavelength readings is usually much
smaller than in the previous cases and required wavelength
resolution is much lower (e.g., m=20, and .DELTA..lambda.=20 nm),
but the principle of the invention remains fundamentally the
same.
In a slightly different application, spectral signatures,
characteristic of some particular chemical structures when
illuminated by laser or other radiation, may be measured. In such a
case, the application is similar to that related to Raman
spectroscopy.
WDM ELED Filtering
The filtering effect of the ELED WDM is now further described.
Taking Eq. 41 again ##EQU46## and the equation (Eq. 43) defining
Littrow wavelength ##EQU47## as discussed earlier for
.lambda..sub.L1 =720 nm, .DELTA..lambda.nm=30 nm,
.DELTA..beta.=K.sub..lambda. .DELTA..lambda., and knowing that
K.sub..alpha. =-1 for .alpha..apprxeq..beta. we obtain the
following equation
which provides solutions to the filtering effect of the WDM for the
arrangement shown in FIG. 17 and in Table 7. Assuming that the four
fibers illustrated in FIG. 17 carry the following wavelengths 1300,
1330, 1360, and 1390 nm, and assuming the corresponding fibers 1-4
represented in Table 7 act as both input and output fibers a number
of interesting effects can be observed. The Littrow wavelengths for
each of the fibers 1-4 in FIG. 17 are positioned on the diagonal
running from lower left to upper right in Table 7. Thus it can be
seen that the Littrow wavelength for fiber 4 is 1480, for fiber 3
is 1420, for fiber 2 is 1360, and for fiber 1 is 1300. The table
thus shows that the 1480 nm wavelength traveling in fiber 4 into
the multiplexer will be diffracted back into that fiber in
accordance with the Littrow equation. The same is true for the
other fibers and their respective Littrow wavelengths.
TABLE 7 ______________________________________ FIBER 4 0 1390 1420
1450 1480 3 0 1360 1390 1420 1450 2 0 1330 1360 1390 1420 1 .rarw.
0 1300 1330 1360 1390 0 0 0 0 .uparw. .uparw. .uparw. 1 2 3 4 FIBER
______________________________________
A more interesting example and one that shows the filtering effect
of the WDM is to enter Table 7 at the bottom entry for fiber 3 and
go up to the side entry for fiber I. The intersection of those two
fibers 1, 3 is 1360 nm. Table 7 shows that fiber 3 delivers 1360 nm
to fiber 1 (as well as 1420 nm to itself). Further it can be seen
in Table 4 that fiber 2 delivers the 1330 nm wavelength to fiber 1
(as well as 1360 nm to itself), and fiber 4 delivers 1390 nm to
fiber 1 (as well as 1480 nm to itself) as shown in FIG. 18C. Thus,
for the system where .lambda..sub.L1 =1300, .lambda..sub.L2 =1360,
.lambda..sub.L3 =1420, and .lambda..sub.L4 =1480 nm, and where
fiber 1 is the output fiber for wavelengths input on fibers 2-4,
fiber 1 will pick up the wavelengths 1300 (its Littrow wavelength),
1330, 1360, and 1390. Thus, it can be seen that the central
wavelength of each of the LEDs used in the system is determined not
by the LED but by the WDM. Thus, the filtering of the WDM of the
present invention allows four channels to be compressed into the
space of only 90 nm or just 3.times. the wavelength separation of
the sources used. This effect again is in accordance with Eq. 68
which states that the distance between the Littrow wavelengths for
adjacent fibers is 2.DELTA..lambda..
In the example above where b.sub.1 /d.sub.1 =4, that ratio of core
to center fiber distance can be achieved by using dead fibers
between live ones in the WDM. The system is adjusted so that the
distance between central wavelengths of the ELEDs used is 30 nm and
the Littrow wavelength of the first fiber is 1300 nm. In the case
of ELEDs especially, .DELTA..lambda. may be as small as 5 nm and
the same analysis as above would apply. It can be seen that with
.DELTA..lambda.=5 nm the total spectrum used by the filtered LEDs
would only be about 15 nm.
An alternative embodiment of the invention may use only one LED the
spectrum of which would be sliced into the appropriate narrow
linewidths for multiplexing on a single fiber as seen in FIGS.
18A-C. This is slightly less advantageous than the use of multiple
LEDs the central frequency of which is filtered out and multiplexed
because the wavelengths of the single LED spectrum that are located
to either side of the central frequency will be of lesser power
typically than the central wavelength and therefore will exhibit
some power loss in the system as shown in FIG. 18D.
It should be mentioned that the optimum design for the multiplexing
WDM is not necessarily the optimum design for the demultiplexing
WDM. In other words, ##EQU48##
External Modulation Systems
Applications of an LED based link other than the three to four
channel ELED system described above include a single LED and
multiple external modulator system as depicted in FIG. 19 wherein a
single LED is demultiplexed by a first WDM and each of the
demultiplexed components is run through an external modulator the
output of which is fed to a second WDM which multiplexes the
signals for long distance transmission to a third WDM which
demultiplexes the signals for use at a receiver. In another
application the external modulators may appropriately be sensors
instead of external modulators to pick up, for instance, three
environmental parameters by modulating the light waves in the
fibers. Such a smart skin system is depicted in FIG. 20 showing one
LED and four WDMs one of which demultiplexes four wavelengths from
the LED and passes them to a network of three other WDMs through
sensors located at the intersection of the fibers for multiplexing
by the three WDMs and transfer to a demultiplexing WDM at a
receiver. In this way, the smart skin network can sense a large
area and a variety of quadrants, determined by
(x,y)-coordinates.
WDM Optical Isolation
Referring to the system illustrated in FIGS. 21A and B, it should
be emphasized that it is important to avoid cross-talk between the
output signal .lambda..sub.1 and input signal .lambda..sub.2, as it
is schematically illustrated in FIG. 21A depicting for example a
Raman sensor, or some type of multi-wavelength sensor, where
sensing medium shifts incite signal wavelength .lambda..sub.1 to an
other wavelength .lambda..sub.2 (usually longer). FIG. 21B shows a
sensing probe having two detectors R.sub.1 and R.sub.2 which can
compare wavelengths .lambda..sub.2 and .lambda..sub.3 received from
the medium to be sensed. .lambda..sub.3 may also be used simply as
a reference wave to sense unwanted motion of the fiber. The LD or
ELED, and detector R may also be directly placed in the entrance
plane (without using relay fibers) in FIG. 21B, obtaining a
bidirectional WDM grating splitter using the same WDM geometry as
in FIG. 3.
As seen in FIG. 22, signal .lambda..sub.1 emitted by a laser diode
LD (or LED/ELED) in a transceiver is usually much stronger than the
received signal .lambda..sub.2 due to its close proximity to the
detector R and therefore should be isolated from detector R located
in the same transceiver as shown schematically in FIG. 22.
Low Cross-Talk WDM
FIG. 23 shows a preferred structure of a WDM which prevents high
cross-talk (lens structure is not shown here for simplicity, see
FIG. 3). It is not possible for .lambda..sub.1 from LD 154 to enter
detector 158 because .lambda..sub.1 is dispersed in the direction
of Fresnel reflection. This optical isolation preferably is very
high, down to -60 dB of optical cross-talk, equivalent to -120 dB
of electrical cross-talk. As shown in FIG. 24, a camera embodying
the high optical isolation design shown in FIG. 23 may be employed
wherein .lambda..sub.1 is the image signal and .lambda..sub.2 is a
coded information signal containing for example positioning
information fed to the camera from a remote controlling
location.
Referring again to FIG. 23, .beta..sub.2 <.beta..sub.1 ; thus,
.lambda..sub.2 <.lambda..sub.1. Typical dimensions are
D=100.mu., g=5.mu., d=62.5.mu.. Pertinent grating equations are
##EQU49## (incident angle and diffraction angles are reversible
here.) ##EQU50## for Littrow wavelength, .lambda..sub.L, also
##EQU51## From FIG. 23
where f is the focal length of the lens not shown in FIG. 23. It is
seen that this system has excellent optical isolation, since the
.lambda..sub.1 signal emitted by the source LD 154 is diffracted to
the fiber 156 at diffraction angle .alpha., and the Fresnel
reflection from the fiber front surface is not retro-reflected to
the detector 158. Analogously, the Fresnel reflection from the
grating 159 is not retro-reflected to the detector. (If
.lambda..sub.1 <.lambda..sub.2, the position of the source and
detector should be reversed.) Since d>>g, and D>>d,
power budget is excellent. The tolerance coefficients, H.sub.1 and
H.sub.2, characterizing the signal dispersion broadening, due to
either wavelength shift (for LD) or source linewidth (for LED) must
be calculated. We have, for coupling between sources and fiber,
##EQU52## where
and .delta..lambda. characterizes wavelength shift for the LD and
linewidth for LED (or ELED). Substituting Eqs. 75 and 76 into Eq.
77, we obtain ##EQU53## For .lambda..sub.1 =720 nm, .lambda..sub.2
=810 nm (thus, .DELTA..lambda.=90 nm), and .delta..lambda.=9 nm,
and .DELTA.x.sub.1 =5d, we obtain from Eq. 78 H.sub.1 =0.5, and
superior optical isolation. Analogously, we obtain ##EQU54## The
system general schematic illustrated in FIG. 21A, can be used as a
Raman sensor (or other multi-wavelength spectroscopic sensors) with
excellent optical isolation (that may be further improved by using
GRIN optics in the front of the source and interference holographic
and multi-layer filters) or as transceivers in security cameras,
where the weak output signal .lambda..sub.2 is perfectly isolated
from the strong input signal (.lambda..sub.1) as shown in FIG.
24.
The system illustrated in FIG. 23 can be implemented using a
holographic grating in quasi-Lippman geometry near the Littrow
position. Such a WDM system can realize full duplex using only one
fiber for both single-mode and multi-mode cases. In such a case
either two wavelengths from two separate windows (e.g.,
.lambda..sub.1 =1310 nm, and .lambda..sub.2 =1550 nm), or two
wavelengths from the same fiber transmission window can be
multiplexed. Where the two wavelengths are far apart, compensation
for chromatic dispersion in the lens may be necessary, or the lens
can be replaced by a collimating mirror, preserving the same
fundamental design. The basic architecture of a bidirectional
dual-wavelength (single-mode) data link, based on such a WDM
system, is illustrated in FIG. 25. This system can be applied in
single-mode and multi-mode applications, as well as for LDs and
LEDs.
WDM Coupler
The WDM bidirectional coupler of the present invention is based on
a broadband volume holographic grating with regulated bandwidth (20
.div.200 nm), high optical density (O.D.) as well as high
transmission in the remaining part of the spectrum. For dichromated
gelatin (DCG) gratings, transmission can be high within a very
broad range of wavelengths (0.3.mu.-3.0.mu.) that cover almost all
wavelengths of interest in optical communication and spectroscopy.
Since the grating's Bragg structure is slightly slanted
(quasi-Lippman), Fresnel reflection is rejected, and the system's
cross-talk, C, is determined only by the grating's O.D. (optical
density),
i.e., for O.D.>5.0, C<-50 dB. The system is illustrated in
FIGS. 26A-B where the conventional lens can be replaced by GRIN
lens. One spectral range is diffracted-reflected (.lambda..sub.1),
while the other is transmitted (.lambda..sub.2). Typical parameters
of such single-mode WDM couplers, are d=9.mu., b=125.mu., f=3 mm
(focal length), .lambda..sub.1 =1310 nm, .lambda..sub.2 =1550
nm.
The system's dispersion loss is determined by the following
formulas: ##EQU55## and
For example, for d=9.mu.m, b=125 .mu.m, .lambda..sub.1 =1320 nm,
and .delta..lambda.=1 nm, we obtain .zeta..sub.D =0.97, and L.sub.D
=0.1 dB. Adding 4% Fresnel loss (4.times. times), we obtain L.sub.F
=0.7 dB, and grating loss, L.sub.G, is assumed to be 0.5 dB. Since
the system is highly-paraxial (2b/f<<1), we can ignore
aberration losses and total WDM loss, L.sub.T, is
This system can be used as a WDM (FIG. 27A), WD(D)M (FIG. 27B) or a
bidirectional WDM coupler (FIG. 27C) or a dual window cascaded
system as in FIG. 27D. Since the holographic grating is only
slightly slanted, the grating period is not small, because
##EQU56## where .beta. diffraction angle, for b=125.mu., f=3 mm,
.lambda.=1320 nm, 1/.LAMBDA..sub..vertline..vertline. =31 l/mm.
Such a large grating constant, however, is not problematic for
volume holographic gratings (alternatively, other types of
holographic gratings can be used). It should be also noted that the
angle .alpha..sub.o (see FIG. 28A) is highly arbitrary here since
it is not determined by .DELTA..lambda. wavelength separation.
Comparing the advantages of this system, based on
quasi-Lippman/Littrow volume holographic gratings versus, for
example, Gould Electronics' Biconical WDM Typer Coupler (see Gould
Electronics, Technical Notes Specifying Fiber Optic Couplers), the
present invention offers the following advantages:
1) superior cross-talk (<-50 dB);
2) relatively easy generalization to a large number of channels
(>10), separated by no less than about 1 nm, and not more than
30 nm;
3) very low sensitivity to polarization; and
4) low sensitivity to wavelength shift (system can be used not only
for LDs and single-mode fiber, but also for LEDs, ELEDs, and
multi-mode fiber).
Feature 2) is illustrated in a WDM duplex system combined with WDM
transceiver, illustrated in FIG. 28. The generalized WDM link
illustrated in FIG. 28 consists of six WDM sub-systems, including
two bidirectional WDM couplers and four multi-channel WDMs (FIG. 3)
implanted directly into two transmitters and two receivers. The
system is bidirectional: the spectrum of the second window
(.lambda..sub.1 .about.1300 nm) is transmitted from the left to the
right, and the spectrum of the third window (.lambda..sub.2
.about.1500 nm) is transmitted from the right to the left. The
system can be applied not only to LANs (local area network) but
also to MANs (metropolitan area network) or WANs (wide area
network) since the link's length, L, can be higher than 20 km. Such
a system can have three or more times higher bandwidths than the
state-of-the-art, significantly reducing the cost of electronics.
For example, instead of transmitting 240 Mbps within a
single-channel, we need to transmit only 80 Mbps per wavelength,
still preserving the aggregate bandwidth of 240 Mbps, since
80.times.3= 240. Referring to dispersion requirements, for
.DELTA..lambda.=1 nm, we obtain total dispersion on the order of
100 ps for 2 nm separation (between three channels .lambda..sub.1
', .lambda..sub.1 "; .lambda..sub.1 '"; or .lambda..sub.2 ',
.lambda..sub.2 ", .lambda..sub.2 '") and L=20 km. This is
acceptable for the majority of MAN (WAN) and LAN applications. Most
state-of-the-art buildings utilize single mode fiber and therefore
MANs and WANs may preferably be single mode.
Feature 4) is a consequence of low dispersion, due to the fact that
grating resolution of bidirectional WDM couplers is low, only
.about.30 l/mm. Indeed, assuming typical multi-mode fiber,
62.5/125.mu., we obtain b/d=2, and assuming a broadband LED source
with linewidth .delta..lambda.=100 nm, and .lambda..sub.1 =1320 nm,
we obtain from Eq. 82 L.sub.D =1.5 dB. Therefore, this WDM system
can be used not only for single-mode fibers and LDs, but also for
multi-mode fibers and LEDs or ELEDs. Of course, this system also
can be used for multi-mode LD optics.
The WDM system illustrated in FIG. 26A is related to the
multi-window WDM described in U.S. Pat. No. 4,926,412. The
reflection part of FIG. 26A can be replaced by that of FIG. 3 in
that patent, while the transmission part of FIG. 26A remains
without change. In such a case, we can demultiplex multi-wavelength
channels from one fiber transmission window in reflection mode and
a single wavelength channel in transmission mode from another (or
the same) transmission window. However, contrary to the
dual-wavelength approach (that must be single-window, since the
grating's wavelength spectral characteristics need to cover the
entire spectrum of interest) the reflection part of the spectrum
need be covered only by the grating's spectral characteristics
while the transmission part need only be placed within a low
material absorption region of the grating spectral characteristics.
This second condition, however, is easily satisfied for all
wavelengths of interest, since for DCG, the low material absorption
spectral range is very wide, from 0.3.mu. to 3.mu.; i.e., it covers
near UV, visible and all three fiber transmission IR windows
located in the vicinity of 0.8.mu., 1.3.mu., and 1.5.mu..
WDM paraxial geometry illustrated in FIG. 3, either single-mode or
multi-mode, can be interpreted in a slightly different way, namely,
instead of a number of output fibers demultiplexing
.lambda.-channels, only one output fiber may be used, located near
the input fiber, similar to FIG. 26A. Instead of locating the fiber
array as in a stationary multi-wavelength system, a single output
fiber is used and the lens is rotated with the grating to adjust to
a suitable wavelength. Eq. 81, with variable b, adjusted to
specific wavelength .lambda..sub.1 may still be used. Thus, a
tuneable WDM filter with variable .lambda.-resolution
(.DELTA..lambda.=0.1.div.20 nm) is obtained based on the same
principle as the stationary WDM system of the present
invention.
Dual Wavelength Systems: Raman and Dye Sensors, Cameras, Optical
Guidance
Yet another application, based on the same principle, is to use the
WDM system illustrated in FIG. 26A, as a dual-wavelength system. In
such a case, we need to use multi-mode fibers (thus, b/d.about.2),
and .delta..lambda. in Eq. 81 can be related to the spectral
linewidth of the broad-band source (LED, for example) and Eq. 81
can be modified to the following form: ##EQU57## where
.delta..lambda..sub.o is spectral linewidth of the broad-band
source, .lambda..sub.1 is the central wavelength of the
diffracted-reflected spectrum, and .xi. is a modifying factor
depending on the direction of Fresnel reflection (.xi.>1). In
this case Eq. 80 holds, and optical cross-talk (or optical
isolation) can be very low. The geometry illustrated in FIG. 26A
can be modified in a number of ways by, for example, exchanging the
inputs with outputs and vice versa, similar to those architectures
illustrated in FIG. 27.
A particularly interesting geometry is shown in FIG. 27C and is
related to the general architecture illustrated in FIGS. 21A and
22. As a specific scenario, we consider a LED signal, with
.lambda..sub.1 =600 nm, and .delta..lambda.=100 nm, introduced to a
sensing medium (see FIG. 21A). We assume that the sensing medium
shifts the LED spectrum to .lambda..sub.2 >.lambda..sub.1. Both
.lambda..sub.1 and .lambda..sub.2 signals return through the same
fiber and the .lambda..sub.1 signal is directed back to the source
(following the reversibility principle) while the .lambda..sub.2
signal is received by the photodetector. In the majority of
applications (communication links, environmental sensors, medical
sensors, etc.) the .lambda..sub.2 broad-band signal containing
information is very weak, much weaker than internal and external
.lambda..sub.1 signals (e.g., -60 dB below them). Therefore, low
internal and external optical cross-talk, or more specifically,
high optical isolation of the .lambda..sub.2 signal from the
.lambda..sub.1 signal is obtained (in such a sense that
.lambda..sub.1 signal is not received by the photodector) and is
essential to the success of this type of device. The applications
of such an embodiment include Raman sensors, dye sensors, security
cameras, FOG-M systems (fiber-optic-guided missiles) and many
others. Fortunately, the optical isolation is defined by Eq. 80
where high O.D. can be preserved by a combination of holographic
edge Raman filter, multi-layer dielectric filter and absorption
glass, as well as a slightly slanted fiber entrance interface. For
example, for O.D.=6.0, we obtain C<-60 db, and excellent
result.
Also, dispersion loss can be quite low. According to Eq. 81A, for
.xi.=1, b/d=2, .delta..lambda..sub.o =100 nm, and .lambda..sub.1
=600 nm, we obtain L.sub.D =1.5 dB. Moreover, according to FIG.
26A, there is no optical connection between the two left-hand
fibers, with respect to the .lambda..sub.2 signal. Therefore, if we
consider the .lambda..sub.1 signal as the incident beam (i.e.,
reversing the direction of the .lambda..sub.1 signal in the
left-bottom fiber in FIG. 26A); the unwanted part of the
.lambda..sub.1 incident beam (the part coinciding with the spectrum
of .lambda..sub.2 signal) cannot be transmitted from the
left-bottom fiber to the left-top fiber or from the left-bottom
fiber to the right-side fiber, thus preserving excellent optical
isolation.
WDM Network
A WDM network, based on multi-mode WDMs of the present invention,
is illustrated in FIGS. 29A and 29B. It includes transmission of
five wavelength channels, .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, .lambda..sub.4, and .lambda..sub.5,
bidirectionally. The system is multi-functional and can operate
with standard digital and analog electronics at the same time. It
also includes other standard fiber optic and electronic components
such as switches, lock-in amplifiers, various sensors, printer,
etc. The WDM network is only exemplary and combines transmission of
voice, computer data, sensor data, etc., with full transparency
(i.e. with low cross-talk, <-20 dB).
WDM Dispersion Compensation
All the above WDM systems, based on grating dispersion, cannot
tolerate very high k-coefficients (see Eqs. 1 and 38), close to 1.
Therefore, for high k, a dispersion-compensation WDM system used
previously for holographic imaging applications as in Collier et
al., Optical Holography; see also R. Kim, S. Case, SPIE Proc. 1052,
(1989) may be applied.
A dispersion-compensation WDM system of the present invention is
illustrated in FIG. 30. The multiplex fiber 160, transmitting
multiwavelength signals, .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, is pigtailed to the GRIN lens 161 (that can be
replaced by a conventional microlens). The optical beam transmitted
by the input fiber 160 is collimated by the GRIN lens 161 to an
expanded wavefront 162. This wavefront is slightly divergent, with
the divergence angle .delta..alpha., in the form ##EQU58## where
NA' is the numerical aperture of the input fiber, and C' is the
collimation ratio of the GRIN lens 161. The collimated beam 162 is
incident upon the first reflection grating 163, that is a
reflection grating (DCG, photopolymer or photoresist). The beam 162
is diffracted into the wave 164, and then diffracted again by the
second reflection grating 165. Both gratings 163 and 165 compensate
for grating dispersion and also compensate optical wavefronts by
compensating for variations in the shape of the wavefronts. If the
gratings are uniform, their grating constants
.LAMBDA..sub..vertline..vertline. are identical. If they are in the
form of HOEs, they are phase-conjugated. Because dispersion is
compensated for due to the preferred use of two conjugate gratings,
the divergence of the second diffracted beam 165A is the same as
that of the beam 162, although the spot size increases, i.e.,
D'''>D''>D'>D. Although the system in FIG. 30 is a
generalization of the system in FIG. 3, its advantage is that it
can couple the output beam 165A without loss to output fibers
(166', 166'', 166'''), even if the fibers 160 and 166 are
identical. In such a case, we need to satisfy the following
condition: ##EQU59## where C'' is the concentration ratio of the
output GRIN lenses 167. Therefor C''.noteq.C', and GRIN lenses can
not be identical. In another version, reflection gratings 163 and
165 can be replaced with transmission gratings. Also, input fibers
can be replaced by laser sources (LDs, LEDs, ELEDs), and output
fibers by detectors.
The excellent power budget and high power margin of the data link
of the present invention is useful in situations where untrained
personnel are used to hook up the link. Additionally the low cost
of the specific light sources (CD LDs and ELEDs) allows the
economical implementation of highly redundant systems having other
sources and fibers. If one source dies the system can be switched
to an existing alternative source. The data link of the present
invention may also be used for high hierarchy secured channels. If
a three channel system is in use, two of the multi-wavelength
channels can be used for communication and the third wavelength,
which would advantageously be located close due to the fact that
various fiber guided waves with different wavelengths have
different space locations in the fiber to the cladding can be
monitored for tapping by unwanted sources.
WDM Coded Security System
In another security system, based on the WDM system of the present
invention, WDM security coding may be used in such a way that the
information signal is redistributed (by using TDM, for example)
between a number of .lambda.-channels that can be randomly located
in .lambda.-space according to a predescribed coding procedure and
WDM hardware built accordingly. In order to break the WDM code, it
is necessary to read all .lambda.-signals at the same time. This,
however, requires satisfying two hardwave requirements: the
receiver must have (A) a WDM-WDDM system identical to that of the
user, and (B) a fiber dispersion compensation system identical to
that of the user (requirement (B) is not necessary if fiber
dispersion is small). In order, however, to satisfy condition (A)
it would be necessary to prepare thousands of WDM-WDDM systems,
satisfying all possible .lambda.-combinations and immediately
install a proper system, with proper .lambda.-locations. This is
nearly a practical impossibility and thus the WDDM Security System
may be virtually unbreachable.
Embodiments of the present invention not disclosed herein are fully
intended to be within the scope of the claims.
* * * * *